Law firms are not co-ops. They are partnerships where a small number own and control the company.
Parent is probably talking about "BigLaw" (thousands of lawyers) and boutique law firms (hundred lawyers or less). Most lawyers at the firms are employees (i.e. the associates) and not part-owners (i.e. the partners). They are not co-ops.
Even in a tiny local law firm of 5 partners/owners and no junior associates, they'd still have the staff paralegals and secretaries as non-owner employees so they're not really co-ops either.
You could certainly have a similar model for software co-ops, using contracts for various non-central parts of the work.
Most worker co-ops also include a probationary period for new hires before they become part owners (because to do otherwise would be a bit insane). A really long probationary period with a low chance of conversion starts looking a lot like the lawyer model...
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.