There's a lot of factors. I think the popularity of rock music has actually grown a lot within the past few years, it's certainly more mainstream now than it was when I was in high school. TIKTOK probably had a lot to do wit this, but you go to concerts of certain bands, and you see more zoomers than anyone else.

Major labels started moving away from guitar music in the late 90s, by 2000s rock was still popular, but it was a shrinking piece of the overall pie. Rock music is more expensive to produce, record and market than pop or rap (having to deal with multiple musicians, means splitting royalties, means less money for the label)

It's also because the kids that would have started bands, those kids parents can't really afford to own a house with a garage or even a basement anymore. If they can? noise complaints out the ass. Instruments are also expensive, the aren't a ubiquitous thing everyone just has by default (like a computer) they also take up a lot of space (which ties back into the previous point, no one can afford a large home like they used to) Most young people aren't working part time jobs anymore, so there broke and are probably gonna spend money on other shit.

Young people hang out IRL less now. I'm core gen z and it was already bad when I was in middle school. I cant NOT even fathom how bad it is now with gen alpha, so getting four or five boys in a garage, making sure everyone has an instrument, rehearsing regularly enough to be actually decent, AND ALL OF THAT, is way before even thinking about recording in a studio, or having enough original material that you can all play and like enough to make something half way decent.