I believe this is the reason:

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".

No BigCorp would ever use a SaaS product that doesn’t have SSO federation. No IT department wants to keep track of individual logons.