This answer likely betrays my lack of imagination, but I'm not sure what Google would use GPT-3 for. It's probably much more expensive than whatever gmail uses to predict text, and the additional accuracy might not provide much additional value.
Maybe they could sell it as a service, as part of GCP? I'm not sure how many people inside Google have the ability to sign $15M checks, you would need at least one of them to believe in a large market, and I'm personally not sure there's a large enough market for GPT-3 for it to be worth Google's time.
This is all to say, I don't think you should draw the conclusion that Google is either stupid or hiding something. They're likely focusing on finding better architectures, it seems a little early to focus on scaling up existing ones.