> Can the EU for some reason not enforce its privacy laws on Uber if Uber keeps its data somewhere else?

Yes. Even assuming these laws still work if data is in another jurisdiction (prob. not), they become unenforceable. If someone sells your data in, say, Somalia, how could EU gather evidence and start a legal process?

> Can the EU for some reason not enforce its privacy laws on Uber if Uber keeps its data somewhere else?

Maybe not, especially if they are separate corporate entities. Uber EU may choose to pay for operation of data storage by Uber US. Uber US is not under the same privacy restrictions and sells the data for profit, then what? Who sues who and for what?

This is also partly about governments - the US in particular is known for compelling access to servers that are on its soil and doing large-scale spying (not that EU powers don’t do the same, but bear with me). Companies operating in the US may not be legally able to guarantee data privacy. So having the data not enter US jurisdiction in the first place is considered safer.