We don't roast anymore
In my high school, kids were brutally honest. Whenever I did something foolish, other kids would make sure I knew. Taken too far, this becomes bullying. But I think the level of criticism I received in high school was net beneficial for me. Not all of the feedback was worth following, but it was always useful to know what people honestly thought.
As I’ve gotten older, I’ve noticed that criticisms have gotten rarer. In college, flattery became more common than criticism. Post-college, it's become unusual to hear any form of negativity cast upon someone in the same room. Giving unsolicited feedback has almost become a sign of social ineptitude.
I think there’s many reasons for this, but the main one is that as you get older, you become less “stuck” to people . Most kids in high school have a few friends that they do everything together with, and they know they'll be together for 4 years. So when someone in the group has a character flaw, it's easier to make them fix it than to ignore it. But as you get older, you develop multiple disparate friend groups, and forced social interactions become more rare. So when one of your friends has a character flaw, you can opt to avoid doing activities with them where that flaw poses a problem. You can just enjoy the good parts of each friend you have.
I think this is an under-appreciated reason that adults tend to calcify their ideas and habits. No one tells them when they’re wrong because it’s not worth it .
So if you want to keep improving like you did in high school, what can you do about this? I think it's a hard problem. Explicitly asking your friends for feedback may help, but not necessarily. I once overheard an executive say, "when someone asks me for honest criticism, I never actually give it to them. Unless they work for me, there's just no value in it for me." So does this mean the solution is to ask your manager for feedback? Probably not; it's never worked for me. As long as you do your job fine, your manager doesn't have much incentive to give you criticism either.
I can think of only two partial solutions.
The first is to improve at reading people. This is expected and obligatory, and may actually be another reason why adults criticize less than children. They expect you to notice increasingly subtle hints, so that they never have to say anything directly.
The other is to find a committed romantic partner. This creates a situation similar to the high school friend group. Your partner has to deal with your flaws every day, and may even be judged for them. So they're incentivized to actually fix you.
I don't find either of these solutions really satisfying. Sometimes I wish people would continue to spout their thoughts unfiltered like they did in high school. Fortunately by being aware of the problem, you can avoid calcifying too early.
 I originally thought the main reason is that high school students hadn't yet realized it's not worth it to fix people. I no longer think this is the reason; in high school, fixing people often is worth it. Kids are quick at learning what behaviors work; what takes time is unlearning behaviors that have stopped working.
 A surprising implication of this is that giving someone criticism is a powerful statement of loyalty. It means you intend on spending enough time with them for them to be worth fixing. Of course, this positive message may not get through.