Currently, one of the custom off-topic closing reasons is:
MathOverflow is for mathematicians to ask each other questions about their research. See Math.StackExchange to ask general questions in mathematics.
Moreover, the second Math.StackExchange is a link. I think several times I chose this link instead of the migration link because it looks like this should be the migration link.
So my reason for this "question" is two-fold. One reason is to make this visible in case others close for this reason instead of selecting migration (which involves saying it should be on another site and then choosing MSE).
The other is to ask why one would use this reason to close instead of migration? The not research level off-topic reason should be sufficient to close HW level questions. Non-HW questions are surely welcome at MSE (or else they can reject the migration).