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?
Yes, you recognize the fact, but you haven’t internalized its implications. You can only have a universal present in a universe of shared time. Ultimately, “the present” is something applicable to and that exists within the mind of a single observer.
One of the hallmarks of science is that different people can independently measure something and confirm its existence. If no two observers can ever agree on what constitutes “the present,” then how can “the present” be said to exist at all? It’s a fundamentally unscientific concept.