The US is supposedly a fair and competitive market.
McDonalds and Burger King know if they make bad burgers their customers will eat elsewhere. Why then, when approval of both parties is so low, does neither party worry about losing votes to a 3rd party?
Because the system is not fair or competitive.
Voting third party is a waste voted, but need not be. In many countries, it isn't.
In an Instant Run-Off election, voters rank candidates by preference. If their 1st preference cannot win, the vote transfers to the next preference. It's like saying "My favourite party is X, but my favourite of the major parties is Y".
Mr. President, I challenge you to allow voters to vote for the candidate they believe is the best, not the lesser of two evils
Or are you all afraid of a fair competition?