Primarily, you want an apples to apples comparison. A law firm is a service provider, however not a consultancy. The service the two types of firms provide are not directly comparable.

Furthermore, a law firm is a place where your assertion - “ discrepancy goes to equity partners (who aren't those doing the execution work)…” Senior partners are pretty critical in bringing and keeping clients.

See https://finance.yahoo.com/news/why-law-firm-isn-apos-0514053... . Valuing law firms is not that straightforward, and profit margin numbers are not defensible.

I would appreciate the source you are basing your arguments on.