I think they’re doing it because of your exact behavior: one-click unsubscribe links are easy to do even if you’re on mobile and aren’t giving the process your full attention. Making you enter your email is a barrier.
They already know your email, I don’t see why getting it again would sell it to a new vendor. Clicking an unsubscribe link already verifies you are a real person.
Very true, the act of unsubscribing itself signifies that the email is still live; more bad faith. As to why not sell it to a new vendor, because that would allow them to check a box that says “we offer a feature that allows users to opt out of data sharing agreement with the partners defined in the TOS and onboarding process.”