If a non-native child lives in foreign language environment use wrong pronunciation/grammar, he would be corrected by friends and teachers. But adults won't get that feedback often.
Example, one of my colleague is non-native. He speak almost perfect pronunciation/grammar, yet he has some minor quirks time to time. I won't correct him because his language skill is more than adequate for the job and we are engineers, not the language teacher.
If I correct every minor quirks of him, he got a feedback, so in long term, his language will be improved further. But why do we have to do that when it has been more than enough already?
Another example, I'm not a English native. So my writing still contains grammatical errors. But as my language skills are improving, I rarely got a feedback these days for my writing become good enough.
Sometimes, I got a hint of feedback like : "Wow, I didn't notice that you are non-native.". People say that because they noticed unnatural writing of mine. I guess that's a polite way of saying "You failed."