Of note is that recently, top US undergrads (in physical sciences) are actually going abroad for PhDs. I think at a statistically significant rate but we’ll have to do surveys.
There are simply not as many positions available in the US.
But in our case, one student actively turned down an offer from a top-3 private university in the US (ranking in our discipline) for a UK university. He specifically cited the funding landscape, since that university had had many of its grants canceled at the time, but of course did not rule out returning post PhD. He was a top student!