It's too easy and they refuse to change.
It's not just "easy". Fax is as secure as the phone network we pretend is secure, so if you act on a fax which appears to come from a specific phone number then you have some level of legal protection from liability. If you use a website or email then you are only as protected from liability as your identity verification system.
My monthly bank payments are electronic, but a few don't have bank account destinations, so it gets done via the bank's paper check service.
If I need to deposit a check, I take a photo of it with my cellphone using the bank's app and it gets processed just fine. The MICR font is highly OCRable, so as long as what else is written/printed on it is legible, everything works well. Even if a human has to review it because it was handwritten, they will only have to briefly glance at most checks. The only thing I actually write checks for any more is my rent. The paper check costs me very little and they cost nothing to deposit on the other end. I think the landlord is depositing them in person, because they seem to do them two or so at a time.