You send me an invite. I choose to use it with my account that I set up while working at your competitor. Now your competitor potentially has access.

At least the way they do it now gives some control to the org as to who has access to their account. It's inconvenient but it biases towards making on boarding hard to make control and off boarding easier.

Also, GitHub wants to avoid the same person having multiple accounts. Their focus is the developer, not the org. So they want all your work to be associated with you.

I still have stuff on my account that is associated with Netflix and reddit. Because the account is tied to my personal account, those orgs can never "remove" my contribution.