It can change, it does change and it should change for that very reason. The fact that you ask this question simply says that it changes so slowly that you haven't noticed it in your lifetime.
Lets look at some common objections:
We tried engineered languages and they didn't get wide adoption.
True, but they didn't work because you asked people to learn a whole new language instead of gradually changing their own.
Languages are inherently evolutionary systems.
True and they will continue to be. I'm not saying we should prevent languages from mutating naturally. But evolutionary systems tend to jump from one local extrema to the next. Over time this leaves a lot of useless and unnecessary peculiarities. There is no harm in trimming those.
There is no central authority that defines a language.
False. For languages that are official for only one country, there usually is a government body (the ministry/academy of education or equivalent) which describes what is proper grammar and vocabulary. It is the case even for some languages spoken in multiple countries. For example, Spanish has Real Academia Española.
Granted, for historical and cultural reasons, English is a bit more of a special case. Still, you can say that there are multiple authorities on it. Think about it - you do have public school English textbooks. Someone has to define what should and shouldn't go there. So in a sense there are central authorities for British English, American English, Indian English and so on. Most people would also agree that the Oxford dictionary defines the English vocabulary. If you don't, I'm interested in how use to settle your scrabble disputes!
Me red hole Okford diktiounery, yo. Dere no "swag" 'n stuf dere, yo!
Sure. There are slangs, jargon and dialects. As time passes the authority may or may not include them depending on usage and other factors. This doesn't mean that the canonical form of the language doesn't exist or that it isn't defined by said authority.
Kids are amazing at learning languages and once you are an adult, you no longer have problems with your native language. So it's a non-issue.
Kids are indeed outstanding considering that all we do to help them start is point at objects and make sounds over and over again. They still make a ton of (what we consider) silly mistakes both in speaking and writing. 350
Imagine if 10% of that learning potential was focused at something else as early as possible.
As for adults - just ask a person with degree in philology. Also misunderstandings happen all the time with roots in language.
So what are the actual issues preventing the constant change of languages?
The main problem is the cost of the change.
New textbooks have to be printed. The public has to be informed and adoption takes time. Old documents have to be revisited or rules to interpret their meaning has to be defined. There are other case specific costs. For example if we decide to add/remove/change a letter in the English alphabet it will be a tremendous challenge in the digital age. Every related standard and implementation, be it character tables, fonts, operational systems, programming languages, hardware like typewriters and keyboards and what not will have to change as well.
The cost of migration after a non-backwards compatible change is a well understood pain in software engineering. Different schools of thought exist on how much and how often is acceptable to introduce such changes depending on context. I could write pages on it, the gist of it is that we have well tried strategies to assess if it's worth doing and ways to mitigate said cost. And my ¢2 is that we should do it more often and in greater quantities than we do today for the natural languages I'm knowledgeable in.
The other problem is social.
Paradoxically, most people don't consider their language simply a communication tool. They see it as part of their identity. To change it is to strip pieces of their identity and to acknowledge that there was something wrong with it in the first place. Some go as far as to call the language sacred. Perfect. Immutable.
As this is worldbuilding SE, I propose that you simply change the culture around it.
Lets say that every day you cross the same intersection. As you are waiting for your turn as usual, you get an epiphany. What if instead of how it is now, the periodically of this traffic light is changed in this specific way? Both ways will have to wait less and there is no other consideration that you can think of. You go home in the evening and you remember what you thought earlier. You are filled with curiosity and excitement. There has to be a reason why it is as it is. Right? It haunts your mind until you finally decide to write an email/snail mail/whatever to the authorities. A month later you get a reply. 634