Time travel doesn't exist because time travel wiped out the timelines where it did

[Umbran]

"Why didn't you tell us this 200 years ago?"

"We did, and you didn't listen then, either."

 

[GMMichael]

"We did, and you didn't listen then, either."

I feel like the Hitchhiker's Guide to the Galaxy sheds light on this. To quote the entry on humans (from the fandom site):

". . . they actually still think that digital watches are a pretty neat idea, and they spend most of their time concerning themselves with moving about little green pieces of paper."