Besides the reasons already mentioned by others here: not all users are the same, and we’re better off if some of them remain in Reddit. And yet this sort of advertisement is bound to attract people who are at the very least completely clueless (otherwise they wouldn’t be seeing ads), if not worse.

Instead I think that a better approach is to simply use the platform. Create posts, insightful comments, use the voting buttons. Also, discourage people from derailing non-political threads with political content.