Because this is not something that's wrong with Ubercart - it's a problem with *Drupal*. The fix would have to be made in Drupal core. You're welcome to push the issue on drupal.org to try to get a fix into Drupal 5.x, but I doubt there will be another release of Drupal 5.x just to fix bugs like this. I believe this particular issue is resolved in Drupal 6.x, but I haven't verified that myself.

If it's a big problem for you, you could always hack Drupal core like acdtrp did by modifying includes/form.inc. That would be the appropriate place to look to change the ids to classes. Be aware that in doing this you might break some jQuery code in Drupal, Ubercart and other modules if it selects based on id, and you might have to change some of the css generated by Drupal, Ubercart, and your theme. acdtrp mentioned that became a problem when he modified the core...