Why would another email address be useless?

I had email address X (gmail) that I hadn't logged into for a long time. One day I tried to log in to it. Correct password, but Google, for some reason, simply decided there's something suspicious about my login and blocked it. X had Y as the "recovery email", and I had access to Y, and I indeed received an email from Google sent to Y that it blocked a suspicious login to X. However, THERE WAS NO WAY TO USE Y TO GAIN ACCESS TO X. Google simply did not offer that option for X, and I had no idea why.

Google doesn't allow you to recover a Google account using only your recovery email address. Despite its name, the recovery email address is not used to recover Google accounts AFAICT, it's only used to receive notifications about security-related events.