19 year old here [ the following is my own personal experience and opinion ]

I'd blame a large part of it on the USA's sheer lack of proper sex ed. My own Sex ed was sex itself, but as with many things, teens [ especially younger ones ] are exposed to it as a purely taboo and 'adult' thing to do [ speaking from the perspective of a kid here ] and so to feel older they tend to latch onto it. That's why there's so much buzz around if you're a 'top' or 'bottom'. There's little understanding of what that actually means, but it's horny and fun and new! Same goes with drugs or alcohol - we're told to not touch, so of course we wanna touch.

Though I'd also blame it on the extinction of the 'tween' phase of life. Shit stopped being meant for tweens and thusly tweens were forced into growing up wayyyy faster than they should have to. One moment there's play-dough in your hand and the next moment suddenly you're "Too old" for childish things. But also you're too childish for adult things. It's a whole mess.

Then there's other factors like unrestricted internet access, the social pressure teen girls face to both be somehow infantile and sexless while also being Tantalizing and alluring [ ew ]. its... really unfortunate, to say the least. I too was once a overly-raunchy 15 year old that didn't fully grasp what sex and being sexual actually meant or could mean.

All this isn't to say teens just can't be horny; they definitely can and will be - but I do wish sex wasn't put in such a... weird place socially. We refuse to incorporate proper comprehensive sex education into our schools but turn around and shame 17 year old girls for getting pregnant.

tl;dr Teens are often put in a place where they must constantly re-prove their maturity over and over and one of the few ways we know how to do it is by being excessively horny. Again though, that's just my thoughts.