The trouble with a reply that needs postage
The printed card is a lovely idea and a poor mechanism. It lands in a drawer, needs a stamp, and by the time anyone remembers to post it the deadline has passed. The host is left calling people to ask whether they meant the maybe on the card, and the final number is really a well-argued guess.
A graduation RSVP card has the same manners without any of that. It is a reply form on a page instead of a slip of paper, so a guest answers from their phone in a couple of taps, and the answer goes straight onto your list rather than into a mailbox. Same courtesy โ telling the host whether you are coming โ with none of the friction that makes people not bother.