Because they do slightly less harm than the fascists. I'm not even voting for the Dems, I'm just voting against the fascists. It's not a cheap dodge, it's the whole principle. Voting isn't going to give us "good" anytime in the.near future, so trying to use it for that is foolish. The comparatively slow march towards fascism under the Dems gives us time to utilize tools besides voting: mutual aid, community building, workplace democracy, etc.
I think you fundamentally misunderstand the concept of "harm reduction". It doesn't mean "Vote for Democrats because they will reduce the harm caused by fascists". It means "Both the Dems and fascists cause harm, but the Dems cause less harm, and since one of the two is going to win in the current political landscape, voting for Dems reduces the amount of harm added".
The only rational voting strategy is voting for leftists in the Democratic primary, and then voting against the fascists in the general, even if that means voting for the neo-lib. If you want to vote against the fascists while also voting for a leftist, you need to make sure leftists win the Democratic primary. We're seeing that starting now.