I mean, I think most people who say this do think the Republicans are more to blame. It's just that they also think the Republicans are beyond hope so there's no point attempting to make them feel guilty.

Also we as humans tend to assign a different flavor of blame to people who do bad things than to people who claim to be preventing them while not actually doing so. If a criminal is holding a victim at gunpoint and then they drop their gun and a bystander picks it up and hands it back to the thug, we tend to view that as wrong act even though it's dependent on the criminal's earlier wrong act.

Likewise when Republicans do bad things over and over and then Democrats argue that they'll do better but they just hand the same system back over to the Republicans, people are going to be dissatisfied with that. Added to this is the perception that Democratic politicians do this to protect their own political position and preserve what power they individually have, which makes Democratic inaction even more irritating.

>It's just that they also think the Republicans are beyond hope so there's no point attempting to make them feel guilty.

How could one ever hope to make them feel guilty about wanting different things than Democratic voters? They do not want to be your friends, or your neighbors, or any other relationship where they should feel guilty for their voting interests. You might as well ask yourself why you don't feel guilty for leaning Democrat.