Imagine, you work in bigCorp. You have company email address: my-name@bigCorp.com
bigCorp pays for your access to SaaS service.
You switch jobs, your email is revoked/removed. You can not log in anymore.
If there was no 2FA via email - you still can access service with email+password in case they failed to remove your access to specific service.
If all services use 2FA via email - bigCorp has less access problems.
That is also partly related with SAML/SSO lack of "sign off".