Tbf, they have before. Ron Paul for instance was a Libertarian who ran as a Republican and won, and they do run for local offices a lot (at least the Libertarians, never seen a local Green on the ballot), they just also put forth a presidential candidate because if they can get like 5% or 15% of the vote (I can’t remember which) they get federal funding and have to get included in the next debates instead of the debates only being R vs D.

Idk about the other third parties, but the Libertarians are doing exactly that.