I know that almost all tickets nowadays are considered "electronic" tickets, though I've seen that most airlines will charge a paper ticket fee if they have to issue you a paper ticket. Considering that most "electronic" tickets eventually need to be printed into paper boarding passes, what actually constitutes a "paper" ticket, and why do they cost so much vs. electronic tickets?
Keep in mind, a Boarding Pass is in no way like a Paper Ticket. They are completely different instruments and not at all interchangeable.
Also, printed Boarding Passes are becoming less common because of Apps and emailed BP's. I haven't held a printed BP in...well, I don't even remember.
A Paper Ticket is a akin to a bearer instrument, meaning, the paper itself represents value and the airline has to physically deliver the ticket to a clearing agent or convert it back to an e-ticket before they can get paid. This requires manual work by a person and takes time.
The reason airlines charge a fee to print a paper ticket is because they are relatively very expensive to process, compared to an e-ticket. The charge is much higher than the processing cost to discourage printing a ticket. They really, really, really don't want you to get a paper ticket.
To be clear, there are situations where paper tickets/vouchers are necessary and if the agent determines this is necessary, you won't be charged. This has happened to me and it wasn't even clear to the Agent why it was necessary.