Since they’re a little less totalitarian than GOP they don’t force everybody to vote with the party as aggressively. When the majority is narrow it like now that means Biden can’t force it through if even just 1 or 2 people aren’t on board.
And then you can look at the track record of the specific senators who won’t go along. The party leadership would have to push alternative candidates to get them voted out to make that happen. But do you want them to have that degree of centralized control?