Two reasons:

1. As you age, your critical faculties improve. When you first discover pop music, it’s like the rush of first love. You’re a blank slate and the music just writes itself into you. It becomes identity. And you immerse yourself into it. And after enough songs, you start getting readier and readier to dismiss things that don’t hit you like that first one. And since very few ever will, the general sense that “music sucks” gets stronger and stronger, until that becomes the default. Also, you’ll start seeing trends go and then come back around, and this will augment a healthy cynicism about the record industry, making falling in love with a song or an artist much harder.

2. You drop out and drop back into what’s current, largely unwilling. The common idea is that people stop paying attention to popular music sometime around age 30. Either the patterns of a grown-up lifestyle (kids, house, IRA) leave you with less and less time to devote to following the latest trends, or you get sick of the trends and the difficulty of finding something that pleases you, and you throw up your hands.

Now suppose some time goes by. Suppose some young people in your life expose you to the next Big Thing. And since your behind the curve on what’s been happening, it won’t sound like anything you’re familiar with. Or if it does, it will sound like an unoriginal or weak repetition of it. So of course the stuff the kids listen to is obnoxious. You’re not in a place to how it got there, and they’re not in a place to hear anything else.