I think it's likely that it's because organisations can manage users by their domain name - it's possible to have a blanket rule that allows users to create an account for a team if they have an e-mail address from that organisation.
That said, I'm surprised they've not let users associate all of your Slack team accounts with a single account.
That said, I'm surprised they've not let users associate all of your Slack team accounts with a single account.