They prefer being in opposition. You get to fundraise off your opponent, you can promise whatever you want and never have to deliver, you don't need a coherent message across the whole party so AOC can be in the same party as Biden and no one will bat an eye, you get to send money to your advertising exec buddies, a whole class of PMC grifters can grow up around you like so much moss, and so on.

Then every once in a while when tensions get high in the working class you get to take all of that and redirect it to within the system and maybe make a small concession or two that will inevitably be undone by the GOP again.