I think because the "mentor" thing is often in the context of iGents learning to dress from the internet (like me). I don't see the difference between IRL and online for such things, but generally I hear people talk about their father or uncle teaching them what is proper as if that influence makes them better dressed.
For a good and nuanced counterpoint, I believe @Academic2 has spoken on this topic.
I am not sure how that is relevant, unless you are (correctly) suggesting that people you know personally are more familiar with your environment and can then give you better advice for dressing appropriately within that environment. Though in that case, it is about choice (you should want to look like this) rather than execution (this is how you look like this).
But in general, environment and context matters a lot less now, and I think that's by and large for the better.
This morning, as I was parking my car, I saw a couple leaving Starbucks and walking toward their car. Guy, 50s, was wearing flip flops and an old, stretched polo and opened the door of his mid 2000s sedan for his wife as if it were the most natural thing in the world.
There are some elements of the "old way" that shouldn't be lost, and for whatever reason, fashion choices seem to be caught up with all that. Men wearing suits to work. That sort of thing.
Or like formalwear.
Don't get me wrong, if the musicians at the opera feel that it is disrespectful for people to be wearing at tshirt under a jacket at the opera, then it is disrespectful. But at the same time, I have a hard time imagining them feeling disrespected if I show up in a navy suit, even without a tie. I know nobody was bemoaning this, but I have seen tears of frustration shed over the death of formal wear.
And at some point, opera singers won't feel at all disrespected by a guy in the front row in a hoody and jeans. I think that'll be great. Democratize the opera.
How would envirionment not be relevant when discussing learned behavior?