Why do the american skaters retire so soon?
Speaking in general . . .
It takes a lot of money to train to elite levels.
It takes a lot of time to train to elite levels.
Talented skaters (and their families) tend to be willing to invest that time and money while the skater is young and hopeful of achieving more success or at least maintaining the same level of success.
At some point, the skater's body gives out, the money runs out, and/or the field gets deeper and it becomes less and less likely to qualify for Nationals much less for international opportunities (both for skaters who have already achieved that level and for those who could only dream about it).
Adolescents become adults. Adults go to college, take full-time jobs, and/or become parents. All those occupations take a lot more time away from training than high school did; it's a lot harder for young (or not-so-young) adults to train to their peak potential than for teenagers to do so. Except for skaters at the very top who can earn more through prize money at international competitions, grants from sports governing bodies, and fees for appearing in shows, it's not prudent to be spending more than many people earn in a year on training for an expensive hobby, even one in which one has had a fairly high level of success, unless one is independently wealthy.
There are sooo many talented, well-trained female singles skaters in the US that there isn't room on the national or international stage for all who might have the skills to deserve opportunities at those levels. It's a lot easier to work through injuries or bad years due to other reasons for skaters competing in smaller fields, who can still take advantage of some of those opportunities even when skating well below their peak.