Moderators can migrate things to any SE site but (and this is rather a huge but) moderators and users of other sites don't want junk posts being sent in. There have been numerous complaints in the past about bad posts being sent over to other sites just because they're more relevant.
Therefore, the only times we'll really consider a migration to another SE site are:
- When it's a genuinely excellent question. These are rarer than you'd assume.
- A moderator from the would-be destination site requests it
- The OP requests it and it has answers. If it has no answers, the OP gets a better service by just posting a fresh post (and having their old old closed) as the date is reset.
... Otherwise we just suggest the OP posts it on that site and close.
To clear some things up...
The complaints I've been talking about were voiced in the Teachers Lounge, a moderator-only chat room. Historically even moderators have pushed bad migrations.
The people who know what's best for their site are the users of that site. The current close-migration pattern puts people who have nothing to do with that site in charge of what gets sent there.
Once migrated, undoing a migration is tedious and involves a lot of work on both sites.
A pull-request system could technically work. One site votes to push a post to another SE site. Once it reaches a threshold users of the other site could vote whether to accept the post.
But this uses a lot of time in both places and doesn't involve the OP.
Moving a question over would require the OP to go to the new site and sign up before the question became theirs again. Without doing this an answer couldn't be accepted (or updated) by the OP. This leads (again, as history has taught us) to abandoned questions... Which waste even more time.
So talking to the OP and explaining which site they need to be on is an essential step that needs to happen in order to get them to engage on the new site.
If you're doing that, rather than go through the very expensive process of migrating, why not just have them post their own question?
In short, closing a question and telling its user why and where they should be is quickest and technically easiest, using the least manpower. More importantly, I believe it's the more effective than shunting questions around.