One reason is because one has to interact with authorities in their countries. They still want us to either use (snail)mail or fax (or both).
For example we mostly email invoices to our customers but we also have to keep hard copies. So we print 3 copies. One goes to customers (eventually) via snail mail. The other goes to our accountant. The 3rd we keep in a folder for when/if the fiscal authority decides to drop in.