Why can't the DNC and progressive voters do the same?
Because the DNC is structured explicitly to prevent progressive challengers.
There are many examples, but one big one is the DNC vendor banlist. The DNC says that any vendors who work with progressive primary challengers will be banned from any future work with the party.
This means that progressive challengers often cannot find vendors to do basic things like print flyers and signs, and finding experienced staff to hire is nearly impossible. And of course, "centrist" candidates have not been bothered by this.
And then at the top of the DNC you have hundreds of superdelegates and party officials who are overwhelmingly center right or rightwing. These people cannot be voted out directly.
UnitedHealth is pure evil.
If anyone ever burns down UnitedHealth facilities or murders anyone in their executive leadership, just know that if I am on your jury you are going to be acquitted.