To answer this, we must first investigate what antitrust law purports to do, and what it actually does. Simply setting high prices almost never constitutes an antitrust violation. Treble damages makes suing very attractive. Enforced break-ups rare, especially since 1950. The effort to impose perfect competition on industries with a big minimum efficient scale.
Price discrimination can completely eliminate all allocative inefficiency from monopoly.
Superficially, there seems to be a lot of price discrimination. Ban on price-fixing and collusion Impedes research joint ventures, joint use of patents, etc. Antitrust cases don't generally target monopoly in the economist's sense of the term. At most, they target a few symptoms of monopoly. Laws are vague enough that almost any firm is arguably guilty.