Because it's understandable and familiar. Everyone just knows how a deck of cards work, so when a player gets lucky/not they don't get upset(where as if you hid it was played as a deck of cards they'd see the game playing against them).

The mechanic is giving a bunch of storable options to a player.

(or sometimes a hidden goal, although technically that is an option that comes available after reaching the goal)

Mechanic of the deck is to bring some randomization.

Have you made a game design involving cards, tabletop or otherwise?

As a lone indie developer with a tiny budget, I find cards much cheaper to produce.

E.g. It is cheaper to order 5 pieces of art (can be just head shots or half body portrait) to make 5 cards that represent creatures, then slide the cards around on screen to represent movement/combat. Than it is to order 5 3D creature models or full body sprites, animate them, and order all sorts of effects for movement/combat.

Then, sell the game as a card game and save a bunch of money and development time. :)

Have you made a game design involving cards, tabletop or otherwise?

As a lone indie developer with a tiny budget, I find cards much cheaper to produce.

E.g. It is cheaper to order 5 pieces of art (can be just head shots or half body portrait) to make 5 cards that represent creatures, then slide the cards around on screen to represent movement/combat. Than it is to order 5 3D creature models or full body sprites, animate them, and order all sorts of effects for movement/combat.

Then, sell the game as a card game and save a bunch of money and development time.

LOL, how true.

But you can make Minecraft it has even less art assets required and still is a bit popular (also I have seen colones that don't even have any textures, just coloured cubes, the ultimate dream for a programmer :D).