Actually most people are not aware. So its better to spell it out.

glances at the current state of the UK Labour party

It's been known to work for a bit, but its also been known to collapse right back into the old two-party dichotomy. I think the hysteria around third parties baked into every election since the Bush Era SCOTUS-powered election theft in Florida is overblown, particularly when so much of the electorate lives in one-party dominant states. But I've also noticed successful outsider parties - the German Greens, France's En March, the UK Liberal Dems - seem to embrace Corporationism as quickly as any of their German Christian Democrat / French Socialist / UK Tory peers.

And then there's always this specter of fascism floating on the edge of the political establishment. Your Alternative for Germany, your National Front, and your UKIP create this existential crisis for liberal voters, such that they're persistently terrorized into voting the "safe" centrist candidates in while ostracizing any candidate actually running on the things they say they want.

The Ruling Elite have the effective roadmap to keep the proles in line. Continuously finance a paper tiger on the right-flank of the election cycle. Make immigration a boogeyman issue that mobilizes the reactionaries within the state to turn out in droves. Then dangle a weak liberal as a release valve - a Starmer or Biden or Macron or Olaf Schultz - that nobody particularly likes, but the liberal-leaning base are told is "electable" because they can win the support of the conservative national media.

People are bombarded with this false choice - weak liberal or strongman conservative - decade after decade, all the way around the edge of the Atlantic, until the institutions these weak liberals are supposed to support are falling apart and the strongman conservatives can easily take over.

Its a doomed system.

Big money donors will never allow green candidates to get into significant office. Money runs politics and billionaires own entire state houses these days

Shittiest take on this community by far.

It's an myriad of reasons from what I can tell. Americans are conditioned to think along the status quo lines even if there is certain degree of freedom of thought. The American corporate media carves the political landscape to intentionally but subtly influence folks to pick either only Democrats or Republicans.

Another reason is that, I suppose rugged invidualism won out in the American society for better mobilisation. As you rightly pointed out, there just isn't grassroots activism among American people (not counting civil and lgbt rights which are undoubtedly grassroots activism and successful ones at that). But this isn't what it used to be. Before and in the early 20th century, there have been other third political parties still gaining respectable number of votes, the last one being the Socialist Party led by Eugene Debbs. He won a respectable 1 million votes as a presidential candidate while campaigning from prison during World War I.

Not sure what happened why political grassroots activism that could counter either Democratic and Republican parties died out, but my guess is that the proliferation of mass media in the 20th century may have had a hand to convince people to stick with two parties, as well as heavy emphasis on individualistic values.

They have a first past the post parliamentary system, derived from the UK. The US has a separation of powers between its executive branch and its legislative branch.

The way to build third parties is by reforming the democratic system state by state to have a ranked choice system open non-partisan primary to select the top two final candidates followed by a general election between these two candidates for each election to elect a representative or president.

It helps mitigate the flaws of the ranked choice system to have it stop at the final two and let the voters choose between these final two choices. It helps get candidates that are at the center of voter opinion distribution.

This means the hard work of mobilizing together and working across partisan lines, recruiting the majority of Americans that are pro-democracy in each and every state.

It's not even psychology, it's just the optimal strategy.

I'm not gonna answer that question. I don't have the perfect answer ready for you.

Instead I will tell you what happens when you vote third party in FPTP. Okay, you have a .nl TLD so I guess ssyou're either in a much better electoral situation or just picked it because it's cool, but I will use the example of the upcoming US presidential election.

Now, let's say the race is really even and it's over. Flipping just one of several key battleground states would've placed Harris in the lead, but unfortunately, Trump won. You look at the votes in your state: Trump won by under 600 votes. Nearly 100,000 people voted for a third party candidate that's actually to the left of Harris. They would've preferred Harris, but because they voted third party, they elected Trump.

If this sounds familiar, that's what happened in 2000. Al Gore could've won. Should've won. But 3rd party candidate Ralph Nader was further left of him and received a bunch of votes that needed to go to Gore. In Florida, he had nearly 100k votes, and the difference between Bush and Gore was literally triple digits. And it wasn't even the only state where Gore lost because of the Spoiler Effect

It's an inherent flaw of the FPTP system and yes, it sucks. It means a vote for a third party is a wasted vote.

Not by voting for people in elections they can’t win. Vote at the local and state level or in primaries for people who will enact voting reform.

Changing the voting system so that third parties are actually possible.

You need a cardinal voting system, otherwise you'll fall prey to Durverger's Law and Arrow's Impossibility Theorem.

I favor STAR, it's the best system designed to date.