These people genuinely don’t understand their own politics at all
Speaking as someone from outside the USA, i have to say that it doesn’t make much sense to me either.
Political parties are supposed to have a fairly equal amount of good points to them that are mostly subjective in nature.
Not this alien “Evil/Good” separation that the USA seems to have.
The real answer is that both American political parties invest enormously into dividing the populace into two coherent groups of voters
The last third-party threat to the status quo was Ross Perot, actual threat, not “guy who takes votes from fellow closer to him”, and election rules got changed on the fly with bipartisan support so this couldn’t happen again
that and to change rules you need to invest billions so you can win by these rules, and then change them so you can’t win by these rules again
see: where’s Trump’s bravado about getting rid of the Electoral College now?