Voting 3rd/independent is an absolute waste of a vote and always will be until that party gets strength in local and then state first. You want a true liberal party to get headway? Got to get those local people voted in. And even then it may take decades. A vote this year for president that's not Republican or Democrat will be wholly ignored.
Local elections have a lot more impact on your day-to-day life. They're just not covered much because the audience for them is so tiny it's not profitable for the media to spend their time on them.
The other reason is that it builds up a roster of candidates that can use name recognition and experience to run for higher state or federal offices. For example, my state rep started off in the town council. (He's not a progressive by any means but it's nice having someone in office I went to high school with and whose kid is in the same school as mine.)
The problem is that at the national level, because of the FPTP voting system, they will still need to be a Republican or a Democrat.
Well okay I didn't consider the entire picture of fptp. Having multiple options to choose from doesn't automatically turn into a two party system, how power is divided after an election is the problem.
Because our first-past-the-post presidential system makes it so that it is exceptionally unlikely that any one not affiliated with the 2 largest parties can get any level of support at the federal level.
If we had a semi-parliamentary system where power was vested in the group that had the most votes in the assembly, you could see more jockeying for third parties at lower levels that still caucuses with one of the two primary parties. But as long as we have separate branches and a system where you have to get 50%+1 vote, we will only ever have 2 parties.
It's more than that though, true leftists don't really have a party in US politics at all. Democrats are still conservatives and it's a shock to liberals when they finally figure that out. And capitalists and lobbyists fight every day to keep it that way.