If identity is established based on one thing you know and one thing you have, the latter should not also be a thing you know because in the sharing economy, we share everything.

https://www.darkreading.com/partner-perspectives/f5/why-facebook-security-questions-are-no-substitute-for-mfa-/a/d-id/1330755?_mc=rss_x_drr_edt_aud_dr_x_x-rss-simple