Faxes have longstanding legal backing and case law. Research points to UCC 1-201(37). Not a lawyer.

Digital certificates could potentially fill that gap but there are technology hurdles in terms of familiarity and ease of use and additional infrastructure. A fax machine can be plopped down and hooked up to a POTS. It’s not encrypted but how many are really listening? It’s a bit of security by obscurity I suppose.

Digital certificates issued by a trusted Certificate Authority (CA) could provide a higher level of security and authenticity. Digital signatures are legally recognized in many US jurisdictions.

I am not sure how many people have MFA tokens to meet the definition of “something you have” to associate to “something you are” plus faxing just works. Juice worth the squeeze?