They have captive voters and are willing to risk Republican rule so Dems can be as rightwing as possible.
It's simply the logical thing for them to do, in the present state of things.
The far left / moderate left voters are captive for the reason you note. The party is closely aligned with center-left viewpoints, so they have their votes. The far / moderate right voters are all voting republican. It's the swing voters / true centrists / center right voters who are up in the air, so those are the people the dems have to appease. So they drift further and further towards their viewpoints.
Unfortunately we can't fix this situation without a healthy show that they don't have the far left / moderate left votes guaranteed, and need to start paying attention to what we want, too... but we can't do that without accepting a republican president, which we certainly can't do right now. So we're stuck. And it'll just keep happening that way, because as dems drift further left, the republicans are just falling off the right end of the scale, and they keep falling further every year.