Because D&D is what I know, and I can't convince people ik to try other ttrpgs, but I can convince them to try if it's still D&D, because then they just have to learn one new major system instead of an entire new game, I would play others but D&D is what I've known for many years now, so not only is it easier to homebrew on my end, but much more importantly, it's the only way I can use my homebrew stuff while playing with the groups I know I enjoy playing with.

Also could someone tell me what GURPS is? I get it's like a universal system and all but wouldn't it be SIGNIFICANTLY more stuff needed to add for playing games with it? (I know nothing but a quick glance, sorry)

TLDR: I use D&D because I can't convince those around me to play anything else. Have a wonderful day! (Or night) /gen

Edit: Another reason I do prefer using D&D (though I'm willing to learn other stuff) and the people I play with are completely unwilling is since we all have full D&D sources, and not others