Some people are passing through and only stay at a city overnight, or are just making a day trip, and getting a card and returning it for a deposit might be a hassle. You might be visiting multiple cities within a brief period, each with a different system and different card, and getting and returning a card at each sounds cumbersome. More or less the same if you occasionally visit different cities in your country or area but not often enough that you'd want to keep cards for a bunch of cities. A local might forget their card and need to make a single trip.

I can see lots of use cases for single-use tickets. All of them are technically possible to cover with a non-disposable card, of course, but that doesn't mean single-use isn't more convenient in some of them.

Nowadays phone apps might also be an option, but that can hardly be the only way of paying for public transit.