Why most of the digital publishers doesn’t provide and option to sign-in or sign-up during checkout?

Some of the sites like The Washington Post, Bloomberg doesn't ask exiting users to sign-in or new users to register during a checkout instead they just ask for the email ID. For new users, yes they'll ask to create an account later but, what happens when an existing user (logged out) uses already registered email id here? A new account is created with the same email ID or that purchase will be linked to that email ID?

I want to know how these things are carried out behind the scenes.