As far as I currently understand it, there are no users other than mods who can vote to migrate a question. Why isn't this ability granted to at least higher rep users, or at least to give users the ability to flag a question (as opposed to voting on a question) for migration instead of them having to create a custom flag message? (This does not include voting to migrate to the site's own Meta site as this is already granted to higher rep uses.)
My rationale to date for resisting open migrations is "because migrations are rubbish":
I maintain those stances. Essentially if we collectively do something wrong like closing or deleting a post, we can undo it. If we collectively shunt rubbish around, we waste that site's time too.
Perhaps more than anything, it takes almost no time to custom-flag:
This is a great question but off-topic, can we move it to Server Fault please?
Two seconds longer than selecting a site from a list? And the mods are more than happy to deal with issues like this when a migration would apply.