The groups writing client software are at least writing open source software, generally under liberal or GPL licensing. So there's that. They aren't selling the software, although they do tend to establish commercial relationships with some entities. Many of those groups work in companies that do something more commercial with other blockchains than Ethereum, though.

Some of the funding for the various implementations comes from Ethereum Foundation (EF) itself. It's not just Geth which is supported by EF funds, but Geth has a special place because of its history.

The Ethereum Foundation takes the view that "client diversity" of proof-of-stake validator implementations is a security requirement: https://ethereum.org/en/developers/docs/nodes-and-clients/cl...

Shipping an endorsed EF implementation would undermine that requirement.

So instead, they publish and regularly update a spec, the major clients are all open source, to some extent the EF has sponsored the development of multiple clients, there is continuous interop testing between all the major clients going on, and the client teams meet at weekly EF meetings.

The next such meeting is today (the All Core Devs meeting), and it's conducted in public, you can listen in real time or afterwards on Youtube. You wouldn't see that if they were ordinary commercial companies.

Ideally they would like to see client diversity in the execution layer as well (Geth et al), and consider single-client (Geth) dominance to be a weak part of the system.

(I am writing an execution layer client that isn't in the list at the moment. Stealth mode, if you like. It's feature is extremely compact storage using a novel database structure, fast storage and network sync, and zk-EVM proofs will probably be coming. The organisation writing it (me) certainly isn't profit seeking with regard to this project (nor paid for that matter; it's a personal side project). Being able to fork and build on other groups' work, as well as test against them, is very useful!)