Why doesn't Signal forbid third party clients or at least offer a client certification program to ensure security?

• Signal forks can have unexpected behaviours like retaining deleted messages and also they don't get updated at the same rate that Signal get updated.

• Every couple of years I hear a story about hackers disturbing signal with backdoors, which would be impossible or very hard to be done If they blocked third party clients. (Ex: 1)

• The amount of people who use third party Signal clients are very few anyway.

I saw what WhatsApp did to forbid modification of it's app which works in stopping a lot of distributions, why doesn't Signal do the same?