How would the phone check whether it's stolen, without a third party, and without the user knowing the password (or lock screen pattern)?
Edit: More seriously, if they took this approach, people would regularly lose their codes, which would necessitate a backup means of obtaining it. Which would no doubt require the purchaser log into their Google account. And we're back at square one.
It might surprise you, but back before games where always online everyone had to keep track of license keys. Still have a box with all of them. Only those that required feedback from activation servers are now useless because the companies killed the servers.
Modern phones are closer to $1000.
(for single player games)