People can say they're running all they want. People can endorse other people all they want. The US has rules about who can and cannot be on the voting ballots; they're different in each state, but they do exist.
Basically, if Kanye was serious about this, he should have done it months ago. Now there's likely no way he can get his name on the ballots in multiple states. So he's telling people to write his name in instead of voting for one of the actual candidates...