What you're saying only makes sense if you think all leftists refuse to vote, or 100% vote for third parties. This is not even close to reality. Libs love to tell me I'm a Trump supporting liar, but I have voted Democrat my entire adult life, including for Harris and in local piddly elections. Not because I think centrist liberal policies are good but because our broken system forces me to, in the hopes that maybe their platform will grow a spine or a conscious and realize caring for your people is worth more than making a buck.
Other nations with less power and resources than us have figured out how to be progressive, what is our problem? It sure as shit isn't the people calling for progress.