There are third parties, they just don’t have any support and the FPTP system is exactly why. If a third party with more leftist ideals form, they’d siphon most of their voters from the Democrats, which means that Republicans would have a bigger bloc and win. So to mitigate the worst result, it is strategically more valuable to vote for the one of the two biggest parties that you hate the least.

Proportional representation and the popular vote would make it easier for more parties to gain influence and in turn get voters.