Because lore and background
The world is deep enough to keep pulling quality content and storylines out of it
People love it anyways
At least it isn't Bleach where it should've and could've ended with the Aizen arc - opinion
But we have asspulls and stuff that doesn't need to exist lmao
It's literally not helping the series and none too many like it
Sure, the DBZ universe is big, but it doesn't excuse the mistakes Toriyama made to his characters and the lore itself.
Gohan, as an example, is one of the characters who suffered from the fans in Japan. Toriyama wanted him to be the new Goku, a new character that could replace him. But when Gohan did something good, the fans were whinning. Thus, with the Buu sage, Toriyama nerfed the character so hard, Gohan just became a joke. He made a bad decision in my opinion, it ruined the lore quite a bit.
Or how about the Buu saga that was never meant to be there to begin with. Again, fans wanted a "follow up" to the Cell saga, wich never needed one. Thanks to that, the Buu saga was confusing, with a ridiculously overpowered villain that destroyed the main concept of Cell (the perfect being), putting Goku back as the main hero and destroying the whole plan of Toriyama to actually make DBZ look fresh in the end.
Yeah, for me, fans ruined DBZ quite a lot.
But let's stop talking about this subject. It always ends up on me talking crap about DBZ and others who are defending it.