Because it will in no way help achieve any of the things they claim to want.
this isn't true, it shows there is a real caucus of active votes who are disenfranchised by the two parties. Whenever there is a breakout 3rd party, there is usually a period of policy realignment in the larger caucuses to pull them back in.
What I think you mean is that it doesn't achieve any electoral outcome, especially one that denies office to a fascist asshole we all oppose. And while that's not an outcome anybody really wants, it does provide an opportunity to bring the democrats to the table when they wouldn't ordinarily be willing, which makes threatening to do so particularly effective this cycle.