The Short Answer Is That Babies Are Just Goated At Phonology. The Working Theory Is That In Order To

the short answer is that babies are just goated at phonology. the working theory is that in order to acquire language as fast as possible, our brains need to be receptive to any and all phonetic information from birth (and possibly even earlier; there's some evidence that prenatal infants pick up on rhythm and pitch information from the sound waves that travel from the parent to the womb). linguists have been testing them on this for a long time, and until they're about 6 month babies can distinguish phonemes from languages they've never been exposed to and who's phonology is completely different to their own. eventually the brain starts to lose that ability in order to focus on correctly articulating those sounds, which is an incredibly complex task, not to mention once you have to start arranging them into patterns to form words and sentences. basically to do takes up lot of cognitive effort that then can't be used to maintain such a massive inventory of sounds.

as for adults, I don't think it's quite accurate to say that it's impossible to learn phonology, since in order to fluently speak a second language you have to be able to understand and produce all the sounds, even if they're not 100% perfect. but in terms of why it's so much more difficult to perfect than something like syntax, it's partly of your brain not being as flexible anymore and, consequently both having a worse memory and a deeply engrained phonemic inventory, to the point that it's difficult for non-native speakers to even "hear" the difference between contrasting sounds your not familiar with (to be clear it's not that you physically can't hear it, it just doesn't register phonologically). this is also why people have consistent accents instead of making pronunciation errors at random; they're still following a set of structural rules, most likely very similar to the ones of native speakers, but with the influence of their first language changing it slightly.

so like. as i understand it, at any age, if your exposure to language is restricted to a single language, you will learn that language. like it just sort of happens, your brain figures out its grammar, semantics, etc, formal training HELPS but is not required. but this is *not* the case with the language's phonology! people will live in a foreign-phone country for years, primarily exposed to its language, they will understand it perfectly, generate it perfectly, and yet will still have a strong "accent" if not trained how to avoid it. that's weird, right? why doesnt the brain learn the phonology? (and why does it learn it perfectly as a baby?) is it too "low level", muscular-level, and that stuff gets "hardened" while higher level stuff is more flexible..?