Because their owners tell them to be.

They don’t even hold primaries anymore because the last time they did they almost lost control to real leftists.

As long as the DNC has neoliberal leadership they will be the good cop in our good cop/bad cop duopoly

Great question. They haven’t always been. Right now, the child (the US voters) have been warned that if they vote the way they did it will be horrible. They believed the grifter, and now they need to get burned a bit to learn their lesson like a child that didn’t believe you that the stove burner will hurt them.

They still communicate like professors and haven’t learned their lessons yet, on average.

Don’t explain verbosely the negative consequences of giving more tax breaks to the ultra rich, instead:

Trump is stealing your money! Trump hates you!

They bury the headlines and communicate to their target audience correctly.