One major difference is that a law firm makes money by selling billable hours whereas a tech company makes money by producing, selling, and distributing an actual product.

You don't see law firms raising millions to open shop, but it's almost a pre-requisite for serious tech companies. A law firm can be profitable from day one. This difference makes it much harder to use the co-op model for tech companies unless all the workers are independently wealthy and don't need VCs.