It isn’t. We really do have a two party system. Voting for a third party that’s closer to your ideology just takes a vote away from the major party candidate that’s the closest to it. We have several examples in the past where a third party candidate cost an election for the closest major party candidate, and zero examples where a third party candidate came close to winning. Roosevelt came by far the closest when he ran independent in 1912 and got 27% (which is why Wilson win).