Virus scanners and other tools like google will fetch any urls in emails to check their target. So having a direct unsubscribe action behind GET of a link breaks your email list as Gmail will unsubscribe everyone
It’s a basic part of HTTP. If you follow a link, that’s a GET request. GET requests are supposed to be safe. A user may not have initiated the action if it’s a GET. If you want something that implies “yes, the user made an intentional choice to take action”, then use another verb, like POST.