So early printer manufacturers made the (quite reasonable) assumption that early adoptors would be nerds, and consequently would be playing TTRPGs. If those early printers were out of paper, they'd show the error message "PC LOAD LETTER" to indicate that they need to be refilled to be able to print character sheets. Of course they couldn't write "PLAYER LOAD PAPER", that would be too long and immersion breaking, so they made the printers act like NPCs by addressing their owners as "PC", and by talking about letters instead of just paper.