If devas are real, than why don't they show us themself? If semigods/devas/dities are their, than why don't they tell us that they do exists, so that people will follow dharma with out doubt?
They are around, just that you don't see them. Likewise bacterias are everywhere, you don't see them.
Not seeing/hearing is not existing obviously is scientifically proven false, but people tend to dwell in this fallacy without remorse.
The way to learn is to improve the capacity. Bacterias can be seen by inventing microscope, but the Buddha seen them 2500 years ago. Maybe one day someone can invent devascope to see them; yet again you can learn what Buddha was capable and see them ?year ahead the inventor ~ :D

- The OP asks for elaboration, I extend my answer -
Dialogue for those who's minds are not contaminated by the neo-Buddhist. (Neo-Buddhists - those who believed in their &quot;scientifically-authenticated-Buddha&quot;.)
Prelude:
Buddhists from tradition and Buddhists at heart are seeing the emerging of the trend of neo-Buddhism, a Buddhism invented by applying so-called scientific methods, surgical skills and dissecting analysis are callously utilized. Taking the convenient standpoint that Buddhism is a religion/(not a religion) appealed to reasoning, unlike Christianity or other faith-based. Therefore, anything that's not scientifically proven should be stripped, for it's either misunderstanding/mis-translating the words, primitive superstitions or later additions by fanatic religious... etc. In a nutshell, they want to reduce the Buddha to be a merely human, any feature of Buddha a Buddha is committing suicidal scientific fraud.
Traditional Buddhist and Buddhist at heart knows the Buddha is the Buddha, yet Buddha is also human therefore it completed the teaching that we humans can become the Buddha, or be liberated forever like the Arhat/Arahant by dwelling in Nirvana.
The blindness of the neo-Buddhist is that they equal reasoning with science. Science is only one of the many methods for searching the ultimate, science is not the ultimate. Reasoning includes scientific methods, but scientific methods not the totality of reasoning. Buddha's teaching is the teaching from the ultimate, he is not searching for the ultimate; reasoning is a method taught by the Buddha to realize the ultimate by oneself, reasoning itself is not the ultimate.
We are seeing science characterized by the ability to re-adjust itself, such as Newtonian physics disproved by Einstein's Relativity. The day will come when E=mc2 being disproved (I said the c, speed of light, is false. How do I know? I learnt it from the Sutras - not directly the Sutra with explicitly this saying but one has to really get it). This is the goodie of science - being able to adjust; the very way science survived is by disproving itself. Applicable is this rebuttal logic used by the Epicurean refuter on &quot;pursuing happiness is the very reason happiness will never be obtained by the pursuer&quot;, likewise science is refuted. We can never predict what science will come up next, but surely can predict whatever come up next will be refuted late; hence a sound mind can infer thus see clearly that &quot;scientific methods&quot; we called nowadays can never obtain the ultimate. Therefore, can we trust scientific authentication? You know better than me. Do you trust someone who come up day after another day, time after another time, telling you, &quot;Sorry, what I said before was wrong, in fact it should be so and so...?&quot;
The Dialogue:
Why don't the Devas show us themselves? I don't think I can answer this question on behalf of the Devas, or as the envoy of the Devas ;). Ultimately one should get his own answer by knowing it directly himself. But I put in my two cents to enrich this investigation:
A) It's really as hinted above, they are around but we don't see them since we don't have the ability to see them, nor it's their duty to let themselves be seen by us.
Here is the incident I read in the Vinaya, that may illustrate this: A Bhiksu/Bhikkhu chopped down certain village sacred tree for building his little hut, the tree Deva's kid just playing around that tree accidentally being chopped off his finger. The tree Deva, devastated, went straight to complain to the Buddha... You see, here this Bhiksu is definitely unable to see, if, for example, he didn't get accomplishment in Dhyana/Jhana. Else he wouldn't chop off the Deva kid's finger, nor it's for the kid to make himself visible in normal circumstance; the Deva didn't file complaint to him directly since he wasn't able to see him.
B) Only with special cause that the Deva to the human can be seen.
A case to illustrate this is from a linked Sutta I read in this forum. In it talked about a Bhikkhu visited by a Deva requested to bring him to the Buddha for asking a question, for the Buddha was surrounded by many higher Devas he didn't have a chance to get near. The reason for this Deva to seek for his help was due to in eons lives before, they were relatives... or something like that.
Therefore, there must be cause (Karma) to fulfill the criteria. Likewise, Aishwarya Rai won't come to see everyone in Mumbai, but she may come to see her friend if she has one there.
C) It's not allowed for the Devas to take &quot;mass-appearing&quot; to induce people following Dharma/Dhamma, for it violated the rule:
(i) Another incident I read in Vinaya, about Maudgalyāyana/Moggallāna being accused faking Dhyana/Jhana for he told the rest that State A would win the battle against State B for he saw the Yaksas (city protecting Deva of State A) winning the other Yaksas in their battle. But it turned out the opposite for while M. reporting this the battle of the Devas made a significant turn...
(ii) Another case I read from the Sutra, said Śakra/Sakka paying tribute to the Buddha for Buddha's teaching turned more people to good hence it increased his citizens decreased the Asura's citizens.
Therefore it shows that the Deva worlds and human world are synchronizing, the battle of the Devas is playing out in the human world, so is the human world reflected back to the higher realm. Imagine, if the Devas made their mass-appearing to induce followers, then the Asuras will do the same, then the battlefield will be shifted to the human world... can the human world sustain such turmoil? (I think the Mahābhārata was such event happened eons ago when Planet Earth was the celestial realm, or semi-celestial realm, before it degraded to be only suitable for humans).
(iii) Last, the Parinirmita-vaśavartin (pleasure from other's pleasure), the highest of the celestial realm ruled by desire, the Lord of this realm is Mara, named Pāpman (Pali: Pāpiya or Pāpimant), his sustaining and joy is produced by the desirous activities carried out in the lower realms, particularly the human realm, he won't allow this mass-appearing, he would prefer humans staying in Samsara, being under his rule.
Ok, it's a bit too sophisticated for it related to ample studies in the Sutras, Vinayas and Sastras to understand, especially many of the sources are not available in English.
The above is very pictorial, vivid, narratively dramatic... for in the tangible it manifested as people, places, events... but in the intangible, it should be decipher differently.
... is it enough now to buy a Big Mac meal in McDonald's, huh?