[doktrin]

For starters, easy access to one of the largest (and in many cases highest paying) job markets in the world. Secondarily, a high quality fallback in the event of significant geopolitical or environmental changes in ones new host country.

In the hypothetical scenario where an emigrant from the US is 100% determined to never return, and who also is confident in the long term stability of their new host nation - then the inconveniences of yearly tax paperwork might outweigh the benefits. However, I think that's more of a corner case than not as evidenced by the fact that the vast majority of emigrants choose to retain their US passport.