Because they oppose what we want with every fiber of their being.
The ruling coalition is united around several things that the people are against. They are skilled at setting up phony issues (or issues that don’t concern the Federal government) to break up the formation of any possible other coalition.
Republicans and Democrats agree that the purpose of the State is redistribution. How much, and to whom, and under what circumstances, there are disagreements. But no elected officials of either Party believe that it is wrong to take from you and give to another of their own choosing, for reasons that make sense to them.
Republicans and Democrats agree that you have “rights” - lots of them. They also agree that any question ABOUT your “rights”, or whether or not something IS a “right”, should not be decided by a political process because that is “divisive”. So, they both agree that the voice of “the People” as contemplated in Articles IX and X can only be voiced by nine unelected life tenure judges, and that five of them, at any time or for any reason, can give new “rights” and take away old ones, particularly if those old ones arise out of majority voting.
Republicans and Democrats all believe in “diversity”. They, ignoring completely the results of all social science research on this subject, and contrary to millennia of human experience and wisdom, believe that the more “diverse” our country, its institutions, and any private entities within her become, the more cohesive and productive we will become.
Republicans and Democrats almost all believe in “free trade” and “immigration”. These things are good for various constituencies of both parties while they wreck the economy and the nation.
Many of the People, perhaps a majority, do not believe in any of these things. But in our existing system, captive as it is to the MSM-mandated “process” for choosing two candidates for POTUS neither of whom will change a thing, leaves the People with no voice.
The people who you call "liberal Republicans" will, almost always, support, caucus with, or vote for a socialist before they will vote for one of us.
That's why, at the end of the day, working AGAINST a third party is futile - because there is no second party.
The Republicans can win if we support them, but we can never win because they won't return the favor.
So we are screwed, blued and tattooed, right?