When I click the "needs improvement" option I don't see any option to migrate to another website. Is this a mistake?
The migration option is under A community-specific reason:
Clicking on 'This question belongs on another site ...' reveals another problem: Philosophy isn't listed in the so-called migration paths:
Only ♦ moderators can migrate questions to arbitrary sites in the network, so you'll have to raise a custom flag, as indicated in the FAQ about migration.
The list is kept short for reasons mentioned here; there's room for three more sites. Based on data from the 10k tools, adding Literature could make sense, but it's a beta site, and historically no migration paths to beta sites are created (see e.g. As of 2020, is it possible to set up migration paths to beta sites?). Other migrations are simply too rare to warrant a migration path.