They are punished because they sprinkle a couple of left things into a mountain of right-wing policies, and keep telling us that we should be grateful. When actually try to push left-wing stuff, they get massive support from voters. When they continuously push right-wing bullshit, they lose support.
Look at Sander's/AOC's approval ratings. Look at the Fight the Oligarchs rallys. Look at the BLM movements. Look at the NYC election.
Then look at the votes for the increasingly right-wing presidential candidates. Look at the approval ratings for those candidates.
Notice a correlation? When they lean left, they get more support from voters. When they lean right, they get less support.
How has moving to the center been working out? If moving to the center has been helping them get more votes, why do they keep losing in what should be easily-winnable elections?