they exist, but the electoral system is designed to keep them from gaining power. And since (most) states use first past the post, secondary preferences aren't taken into account, so if you vote third party when you would have voted for Harris otherwise you increase Trumps chance of winning. There's some argument to be made that voting third party or not voting might influence democrats in the future to change their policy to appeal to those voters but I don't know how realistic that is.