Because some sites/companies force people who do not want MFA to use MFA, and people aren't willing to sacrifice convenience for security like that. (Probably

Because some sites/companies force people who do not want MFA to use MFA, and people aren't willing to sacrifice convenience for security like that. (Probably also because some people really don't get the point of 2FA and demand that feature, but the above is enough of a legitimate reason to support it.)