Hmm that might be it. I am not sure.
If you want to submit these digitally, just call them and ask to see if it is possible. But do keep in mind that you do receive your SSN and passport (and permanent resident card perhaps, I was too young then to pay attention to that) in the mail when you first obtain them. So it's not like these type of documents don't go through the mail on a regular basis.
Same thing with utility bills, tax returns, bank statements, etc.