Q:

Why are dental sounds rare?

Dental stops are rare in the languages of the world (other than in Australia). Most languages outside of Australia containing dental stops belong to Indo-European, Uralic, Kartvelian, and Dravidian. Some languages in the Americas and Africa also have them, but it's extremely rare in both places. Of such, only Dravidian, Indo-Aryan, some languages in the Americas, and maybe Burushaski (if the non-retroflex coronals are dentals) have a contrast between dentals and alveolars/retroflexes (the other languages with two coronal series distinguishing between alveolars and retroflexes). Dental fricatives and sonorants are even rarer. Any reason why are dentals (especially contrastive ones) rare outside of Australia?

A:

I think that part of what makes dental plosives seem rarer than they are, is that when there is no distinction between multiple coronal series (sounds made with the tip of the tongue, roughly dental, alveolar, and post-alveolar), people just write down the alveolar, no matter which one people actually say. For example, while French usually has a dental version of /t̪/, since it does not contrast with alveolar /t/, people tend to write it as /t/ in cross-linguistic comparisons.
I would also claim that part of the reason that the lack of dentals seems odd is that the IPA makes dental, alveolar, and post-alveolar look like the main distinction is in the place of the tongue, but frequently, the real distinction between coronals is in the shape of the tongue (laminal/flattened, apical/thinner, sibilants tend to have a slight groove in the tongue, post-alveolars tend to be with a domed tongue etc.)
You don't frequently see a contrast because a lot of different coronal series are fairly similar in terms of both sound and production. Grooved-tongue /s/ is a lot louder than any other tongue-shape fricative, so tends to be the default coronal fricative. Alternatively, grooving the tongue on /t/ takes energy over a laminal or apical tongue shape, but does not change the sound in an easily noticeable way, so rarely is used.

A:

I don't believe that we have enough evidence to decide whether dentals per se are "rare". In languages with a single anterior lingual place of articulation, it is unusual for anyone to (authoritatively) comment that the stops are in fact dental. The two things that are rare are (a) contrasts between dental and alveolar places of articulation and (b) θ/ð as opposed to s/z. θ/ð contrasting with s/z is reasonably common compared to languages with t̪/t. The reason why the fricatives are more common compared to the stops is that they are easier to distinguish (the spectral properties of [s] and [θ] or [z] and [ð] are very different). In comparison, the spectral distinction between [t] vs. [t̪] are much less noticeable.
The acoustic differences between dental / alveolar fricatives and stops have an effect not only on children learning a language (affecting the survival chances of a distinction), but they also affect the probability that a linguist reporting on a language will correctly report that a language which has a single "t" series in fact has a dental. I do, however, expect that the native language of the linguist reporting influences the description. (This is a well-known effect in Bantu linguistics w.r.t. the transcription of the degree-2 vowels that are variably transcribed as [e,o] or [ɪ,ʊ], even in a single language).

