The reason for digital signatures is that they make a claim. "As a representative of organisation A, the binary with shasum XXXX is our work. We stand behind it." Why would I generate a private key, then share my private key with google? If google wants to claim that a binary they're shipping to users is same the one they received, they don't need my private key to do that. They can make their own signature, with their own key. Using a key I generated then handed to them is just dangerous security theatre. Google is asking me to vouch for binaries they sign and serve. But I can't vouch for those binaries - I didn't produce them and can't make any claim about their provenance.
The reason for digital signatures is that they make a claim. "As a representative of organisation A, the binary with shasum XXXX is our work. We stand behind it." Why would I generate a private key, then share my private key with google? If google wants to claim that a binary they're shipping to users is same the one they received, they don't need my private key to do that. They can make their own signature, with their own key. Using a key I generated then handed to them is just dangerous security theatre. Google is asking me to vouch for binaries they sign and serve. But I can't vouch for those binaries - I didn't produce them and can't make any claim about their provenance.