They exist, but are only really relevant at the local level, or rarely as a spoiler effect if one gets vaguely popular. This isn't so much that people don't want other options, but mostly because the US system is badly designed. The US has a first past the post, winner take all type system, ie, if you win the most votes in a given election, you win that spot and it's all yours. That makes some intuitive sense, but is actually not the most democratic option, as it means that parties that have, say, 20% of the population supporting them don't get 20% of the seats, they get none of them, because for each individual seat, they won't win the most votes. Worse, such a party will cause the major party it is less similar to to win, because it's voters voted for them rather than the major party it was most similar to, so even if the voters on that side of the political spectrum are in the majority, their votes are split among multiple parties where their rivals that stay as one party can then be the single largest one. The US system accidentally makes it mathematically inevitable for two and only two parties to dominate.

Historically they have switched up once or twice, when one party became so unpopular that it basically became nonviable, and a different party rose to replace it, and once the current major parties have swapped ideology more or less, but this kind of thing is very rare.

Because the US doesn’t have proportional representation and uses “first past the post” voting, basically any vote for a third party candidate is wasted.

There are other parties but their main effect is to sway elections by “stealing” votes from the two main parties. There’s a group trying to form a party called “No Labels” right now but if you look at their financial backers, you can clearly see they only exist to try to weaken democrats.

Admittedly there are some other parties that genuinely try to get elected, such as the Green Party, but they rarely succeed in even the tiniest local elections.

The rest of these posts focuses on how insignificant the third parties are, however the real reason is that our democracy was designed as an adversarial democracy, it's been designed so that the two major parties, whatever those two major parties are, are supposed to be in a balance of compromise while also locked into a battle for control. The theory behind this is basically that through the adversarial process a measure of balance would be achieved.

Obviously the design is broken as we're on the road to a second civil war, however that too might be by design as Jefferson, one of the architects of our democracy, can be quoted as saying "The tree of liberty must be refreshed from time to time with the blood of patriots and tyrants" so maybe they just saw revolution/civil war as a natural occurrence in their design.

As for third party replacements, the Republicans, if they lose and keep bowing to trump and losing popularity, are facing the chance to be replaced by another political party, most likely it will be the Democrats that become the conservative party again, as they were prior to the civil rights movement and the Southern Strategy , as many neocon Republicans have left their now far right party to join the much more moderate democrats, so in theory a new Leftist/Liberal party would arise to take their place. So to achieve that first the Republican party would need to continue it's downward spiral, and then a third party would need to pass a certain threshold for votes, once that's done we would have our new two parties.

It also doesn't help that the current most popular third parties, Green and Libertarian, are both heavily funded by both Right wing and foreign sources, so even their marginal successes currently are dubious.

Now with this all said I'm sure someone who's actually got law degrees or political science degrees could explain this better and more accurately but this is as I understand it, overall we have a lot of hurdles to overcome but until the Republican party crumbles away we're stuck with a party that has turned into the Qult of Donald Trump who wants to end democracy completely so that they can have all of the power and control, and that's why we must not focus on third parties but instead focus on beating the wannabe tyrants that want to turn our country into the Trump kingdom.

Yes. Consolidation of power. It sucks. They have found we are easier to control when you just have a bad choice and a worse choice. Easier to divide into “us vs them”. And we have too many people that ate lead paint chips when they were young running things that think this is the only way and refuse to change it.