How about because it's not that simple? Being a teenager is fraught with insecurity, loneliness and the perception of being misunderstood. Lots of their maturity is still cooking.
The teen years are also very formative. So if we can take an extra moment to give them the right nudge early in their trajectory, the final outcome of that tiny investment of time could be huge.
If things about a shared project are crappy, that's worth mentioning, of course. That can and should be done in a way that's constructive and encouraging, though.
I don't like the idea of a community that's too self-important to indulge the ambitious fragility of youth.
I find it enormously fulfilling to mentor younger programmers and have luckily had many chances to do so. But my personal development always suffered at those places compared to the places where I was working mostly with those near my level or above it.
An online community can't serve two masters, it is either mainly for peer discussion (and elitist) or it is mainly beginner friendly. Beginner friendly often wins out because: "won't somebody please think of the children" is a discussion ender.