Let’s imagine a world where time machines are invented.
Hypothetically, what’s stopping anyone from travelling to the past, where the dollar is much more valuable, and buying things at a much lower price? What if you then go back to the present, sell those things at a higher price and repeat the cycle? And wtf would happen if everyone there started doing that?
I think it was Alan Dean Foster who said (paraphrased) that time travel is impossible not because time travel is impossible but because there’s a certain point where all of the time traveling that can be done will have been done.
The only downside is that you never know if you’re on the final loop or not, so everything you do may be the most important thing you will ever do, or it may not matter at all.