Migration should only happen when the site on the receiving end actually wants Thethe question; that is, the question should be good enough so the only thing wrong with it would be where it's asked. Needless to say, that is rare.
Community Managers could set up a migration path between two sites which works like a teleporter, and only then you'd have an option to flag for it. Maybe. Anyway, then, four close votes would suffice to give the verdict of migrating a question, eliminating mod intervention, but since close voters can be reckless, they tend to migrate crap, which just ends up feeling like throwing something into a trash can. The target site won't appreciate it.
On Chem, well, we just don't have too many migrations, and mods can handle them. It's more fun to blame for a rejected migration than to be blamed for it . . .