Part of the reason seems to be that a significant chunk of them have apparently forgotten that antitrust laws and enforcement are hard requirements for capitalism to actually work and deliver on any of its touted benefits.

Unregulated free market capitalism that so many people seem to root for (knowingly or unknowingly) is just a degenerate version of the system that invariably results in cartels, price-fixing, rent seeking, thought policing, human exploitation and suffering, which is really just a different version of the "communist" boogeyman that they fear for the same reasons.