My understanding is that USFS will "embargo" skaters from switching to a different federation for several years if the federation has already invested significant money etc. in their development -- which they will have done if the skater has already earned important international medals that qualified them for A and B funding envelopes. Other major federations probably have similar policies.
From the skaters' point of view, the best time to think about switching countries is before they have any international assignments -- or at least before they win major international medals.
For the young skaters, though, they probably still hold hope that they will become US stars, which would be more prestigious and generally more lucrative. Small start-up federations are not likely to be able to offer any funding, and just traveling to their nationals might be significant extra expense.
Also, of course, fans are not likely to start speculating about skaters who haven't already earned at least some national success.