Makes sense if you consider that the keychain is typically backed up to iCloud, under your Apple ID. So if you generate your Apple ID password through Safari and then lose your device, you’re going to have a chicken and egg problem…
Your Apple ID password is effectively your master password for your keychain. The whole point of a master password is for it to be stored in the brain of the end user.
They probably turned it off after an increase in support requests when people inevitably at one point don't have the "wallet" around. Of course they don't care for other sites support load enough to look at their 50 entries long list of special cases and think "are we doing this wrong".