Because each generation naturally rebels against the one that came before. When Gen Z saw that LGBTQ+ people were the minority in their parents' generation, more and more of them chose to be LGBTQ+ because it makes them "different" and demonstrates their "independance." In truth, most Gen Z LGBTQ+ people are just trying to prove a superiority that never existed in the first place.
Keep in mind, I'm talking about jerks who are LGBTQ+, not the community as a whole. My best friend on imgflip, Mex, is bisexual. I have a few gay friends, and one of my old friends was trans. Most LGBTQ+ people I've met are actually super nice, and I try to be kind to them. What I'm against is people throwing a hissy fit because I was trying to be nice or not offend them. If the trans person I mentioned above had just politely told me that they didn't use they/them pronouns, I would've at least tried the he/her pronouns. Sure, I'd be a bit confused, but I also wouldn't be talking to you right now about how disappointed I am in that person.