I think it is simply that the majority of Democrats are as corrupt and power hungry as the Republicans.
A few, AOC and Sanders come to mind, seem reasonable. I think some, like Biden, are idiots and wont admit the truth to themselves. (See Biden denying american history of discrimination against Asian peoples).
But I think the majority (Joe Manchin being obvious example) are obsessed with power, and realize what the Republicans are doing. I think they are calculating the best way to hold onto their power. They realize if they do the right thing and fight then they will most likely loose. Thus, they are taking the strategy of trying to push right and cozy up to the Republicans so they still have some token position as the Republicans take more control.