Hm, a thought about the Niven Effect (the idea that time travel is possible but never gets invented, because if it does get invented, eventually someone makes a change so it doesn’t)… If time travel is possible, then there are bound to be some other technologies which are also possible, and related to time-travel tech, but which don’t actually involve time travel. In such a universe, would we expect those other techs to exist? Doubtless, often the change that ends up undoing time travel also undoes those related technologies. But then, those techs also have their own set of related techs, and so on. Maybe the only stable universe is one where no technological development at all happens, because any universe that starts down that road eventually ends up developing time travel, and eventually some time traveler will decide to go back far enough to undo all of it.
I’ve seen other reasons. Colonizing the past and hunting dinosaurs (guess the real reason they went extinct) are two from the novel Mastodonia that come immediately to mind.
The Game of Blood and Dust had a pair of aliens play a time travel based game with Earth. They each took turns changing history, with the goal of Dust being to manipulate human history into nuclear apocalypse, and the goal of Blood to keep humanity alive. Not that there was a moral distinction between the two.
"Best two out of three?
"Sure. This time I’ll be Blood, and you be Dust.