But can American companies not hire these Eastern European developers to do this? If yes, why do the US salaries stay an order of magnitude larger?

VCs and senior executives are largely hesitant to randomly go hire from any random country, eastern European or not. Unless there is an established city, like say, Bucharest. Most managers don't want to deal with 1 person in each random city. Having an official office in a big EU city is no brainer. It is being done as well.

Unrelated: A friend of mine had a well established Vodka factory in Crimea/Ukraine randomly taken over by some goons. Sometimes people worry, for good reason or not.

Some can. But hiring people in another country adds another layer or two of indirection.

Anecdotally, software companies that pay the best tend to go the opposite direction: they hire people that are good at self directing and self motivating, which can remove entire layers of indirection.

In some cases, they can. A friend of mine is a project manager for CC Bill, a payment processor for porn sites. They hire almost exclusively from eastern Europe for software development. But if you want to do business with the US government or financial or healthcare industries, i.e. where most of the money is if you're not an ad tech giant, you're largely restricted to hiring US persons. That doesn't mean they need to be citizens, but they need to live in the US, and live here legally, which largely means they can command US wages.

American companies can but the pool is not large enough to open offices there. And if they are opening international offices, it's likely going to be in Asia where there is more money to be made.

If you are in EU, the offices are likely going to be in western Europe or Switzerland or UK - larger addressable markets.