They are not what they appear to be:
Things happen because a small fraction of centrist Democrats side with the vast majority of Republicans, but then the overall legislative vehicle ends up being moved on a party-line vote. This leads to people criticizing “the Democrats” for doing things that only a tiny minority of Democrats actually did, and Republicans run around acting like they have nothing to do with outcomes even though they’ve actually been decisive in shaping them.
That's because those "centrist Democrats" (aka Blue Dogs) are functionally the equivalent of Republicans
, only with more power.