I dont think this is why no one is saying this. But the reason you shouldn’t do this is because of the Efficient Market Hypothesis. This is the same for basically any investment where you are trying to be “smart”, whether you are buying gold, low tech stocks, various currencies, crypto, etc. The fact is, sitting on your ass and clicking a few buttons on an investment website takes literally no effort - which is why there are trillions of dollars in investment funds trying to do it as profitably as possible. Every dollar in the market is competing to eak as much value out of every minute in the market as possible, and these dollars are very smart.

Like, if you graduated top of your class from MIT in financial analysis, you are still at an unimaginable disadvantage, because the evil capitalist hedge funds hired all your classmates, and also all the equivalent graduates for the past 40 years where they have all been competing against each other that whole time. And they have shit tons of money to spend on the best tech they can possibly afford in order to make tiny improvements in trade returns.

You can exchange dollars and euros on the open market, which means the banks and hedge funds can do that too, which means that the anticipated difference between the two is already priced in.