If Jill Stein couldn’t win a majority of left voters via a primary, then how is she going to win a national election?

Stop. You’re hurting their feelings and that’s going to force them to blow their vote on a 3rd party or not vote again, leading to more conservative fascism.

And why can’t these alt-left parties ever start grassroots movements at the local level, instead they only care about presidential elections and just want to skip straight to the endorsement without winning any primaries

You’re confusing them with logic. You’re going to boil their brains. Please have mercy.