I have. What I am saying is that other people are not willing to.
The largest third party in the US is the Libertarian party and if you know anything about libertarians you will understand that trying to get them to agree on something is like trying to wrangle cats : it's a tiring, difficult activity that takes a long time and leads to a lot of screaming, hissing, scratching, and the occasional furball. The Greens are not much better because, being a bunch of hippies and PETA members, they are not liked by most people so only fellow hippies and PETA members tend to vote for them.
The problem is that the third parties in the US tend to be fairly extreme and/or fringe in their ideological platforms. The moderates on both sides of the political spectrum tend to coalesce under either the Democrat or Republican umbrella. To be fair there also used to be a fair degree of variety within those two parties in the past. Franklin Roosevelt was a New York Democrat, for example. That same Democratic party also used to be the Party of the segregationists in the American South. There was not exactly a lot of common ground between those two groups but they still caucused together. This hasn't been as prominent of a thing today as the parties have both regularized quite a bit but the differences still exist in some areas. It was a Massachusetts Republican governor ( Mitt Romney ) who first ordered the issuing of marriage licenses to same-sex couples in the Commonwealth of Massachusetts after the Supreme Judicial Court ( the Massachusetts version of the Supreme Court ) ruled that homosexual couples have equal rights to civil marriages in the state. These are both historical examples but there are still some contemporary situations where internal differences can become emergent.
I'd love for someone to create a more "big hat" political third party that isn't just devoted to fringe or specific ideological goals. There actually was a brief period where I believe the Republican party could have split into different parties over Donald Trump. One part of the GOP loved him and another part loathed him. Had the part which loved him not emerged victorious in their internal struggle I think there absolutely would have been a "MAGA" party created. That would obviously split the Republicans votes in a lot of races and effectively hand control to Democrats if it were to happen which is why a lot of Republicans "fell in line" with Trump : they thought of him as a lesser of two evils between total Democrat control and Trump's own... inadequacies. While Trump is a polarizing figure this was the closest I have seen to the creation of a bigger, more encompassing, less fringe political party. Despite what the media may lead you to believe his supporters are actually relatively diverse in their beliefs so even a "MAGA Party" would still have broader mass appeal than something as specific as the Libertarians or Greens.