You're underestimating the total number of people who just associate themselves with a party arbitrarily and vote down the party line IMO - but even ignoring that I disagree that people wouldn't vote for third parties if they were truly viable alternatives. A LOT of people in the Republican party in fact are repulsed by Trump; and there is also an obvious distinction in the Democratic party between AOC/Bernie-style progressive factions, and centrists and moderates in the party like what Biden represents. The super-crowded Democratic primaries made that pretty clear.
I think the big problem with the system as it stands is that people rightfully understand that if they vote for a third party candidate, that candidate is a) guaranteed to lose, b) guaranteed to be unable to enact any change in government without having a literal majority in the senate, and c) guaranteed to steal votes from a candidate who you may not love but is the lesser of two evils (plenty of libertarians do not agree with Biden much at all, but would rather have him than Trump/vice versa, and then vote down the party lines to avoid disaster).
Third party votes are often cast as a waste of a vote - and I don't blame people for thinking that way, because the system encourages it.
I think the big problem with the system as it stands is that people rightfully understand that if they vote for a third party candidate, that candidate is a) guaranteed to lose, b) guaranteed to be unable to enact any change in government without having a literal majority in the senate, and c) guaranteed to steal votes from a candidate who you may not love but is the lesser of two evils (plenty of libertarians do not agree with Biden much at all, but would rather have him than Trump/vice versa, and then vote down the party lines to avoid disaster).
Third party votes are often cast as a waste of a vote - and I don't blame people for thinking that way, because the system encourages it.