If Homebrew auto-signed third-party code, that puts them on the line for the security of that code. The whole point of MacOS developer certificates is to increase the trustworthiness of the software you run on your machine. The trust comes from the formal relationship between Apple and the software developer, which includes a traceable financial transaction. If signed software proves to be malicious, attribution is trivial.

If the homebrew team signed everything, they would immediately become a target for bad actors. The bad actors would flood homebrew with malicious binaries, which homebrew would auto-sign, users would download & run, and the bad actors would laugh all the way to the bank.