Because the oldest people making decisions don't understand technology and refuse to accept that better ways are available if old ways still work. Fax machine users would rather work harder than smarter.
I received a fax at work once, 10 years ago. The subcontractor has issues with their email so they call me to warn me they were faxing me the document I needed.
It took me a good 2 hours to finally locate the fax machine in the factory, no one had any idea that we still had a fax machine running.
I think you can send faxes to computers and they just get received as emails with attached PDFs. It's not great because of course it's just a picture of a document, so it's not searchable or editable unless you run it through an OCR program.