This is exactly right; any answer to any question about “why doesn’t the left do X thing the right does” is always the same.
My friends and I have attempted a version of taking over the Democratic Party from the left. The campaign against us by the establishment has been unrelenting.
If you're ruthless for the right, you get money put behind you. If you're ruthless for the left, you get money put against you.
America First Legal got >$50 million in funding. A left version of the same would get attacked by Dem orgs and donors, and everyone associated with it blacklisted forever.