One of the biggest problems with antitrust regulation is that the statutes are so vague it can be difficult to tell what is legal and what isn’t. From p. 28 of Robert Bork’s 1978 book The Antitrust Paradox: A Policy at War with Itself:
To put the matter roughly, lawyers forming a partnership could lawfully agree on fields of exclusive specialization (which is market division) and the fees each should charge (price fixing), while the same lawyers, if they were not in a partnership, could not do these things lawfully.
The same logic applies to anything a company does in-house. Hiring an in-house accountant instead of using an outside firm is a form of vertical merger. So is hiring cleaning or cafeteria staff instead of using contractors. More than a century of case law has not settled the matter, at least for companies above a certain size (which also hasn’t been defined). The uncertainty can make companies hesitant to make efficiency-enhancing decisions that might benefit consumers.