It's not just the US. These mainstream political parties are telling on themselves when they don't do what you've suggested. They conceive of themselves entirely in electoral and legislative terms. It's the downfall of 'left' parties the world over. It has never and could never cross their minds to do something with their huge memberships. It's beyond them in every sense of the word.

Communists parties, on the other hand (and anarchist orgs, tbf) start by working with the people. To libs this can only every be a calculated trick to fool the people into support. The cognitive dissonance must hit like a gorilla to hold those thoughts together at the same time in their heads. No wonder they're dazed.

Better vote for the party that is never in danger of tricking passers-by into thinking it might do something to help the people.