Lawyers write laws to aid their professional fellows evade the requirements that affect all the rest of us.
The very large law firms rely to a significant extent on their largeness to gain the confidence of clients who have a lot of money to lose if the firm screws up. When a large client signs up with a 1000 partner law firm to handle a high stakes transaction or litigation, the client requires the confidence that the entity they have legally engaged has very deep pockets that all owners of the entity have a big vested interest in protecting from liability. An investment bank hiring a law firm to structure a $5 billion securities offering, or a major audit firm hiring a law firm to defend it against a $10 billion lawsuit, is not going to hire a legal entity that has 49 employees and legal/financial firewalls between that entity and 50 other entities that are all marketing themselves under an umbrella name. It is going to hire a legal entity that has very deep pockets AND a large and expensive malpractice insurance policy (large enough that an affiliated group of 49 employee law firms couldn’t possibly afford to each have such a policy). Same applies to large accounting firms — with most of the original Big Eight having met their demise precisely because they were each organized with a single primary legal entity, and when one division got hit with a multi-billion dollar liability judgement, the whole firm was liable and went under.