Because thousands of people already have an account tied to a specific email and are going to click the Apple button and get really mad when we can’t log them in.
So then you ask them for their email address and password once and link the accounts together?
And Apple Sign In helps this user, how?...
Apparently they want to use it, otherwise they wouldn’t, right? This way they can have the easy login using Face ID and you can use the account they already have.