Why does everyone assume that this would have won the election for Democrats?

Because genocide supporting centrists keep blaming the anti-genocide left for harris' loss.

There's absolutely way more simply apathetic non-voters who saw two candidates promising more austerity and genocide and said "shits fucked yo" and didn't bother. You can call those people stupid but you'd be dumber for thinking that will get them to vote for your candidate.

If anything the uncommitted campaign was warning about that.