Because of the added simplicity of using a computer to do all the work in keeping the game going. Without a computer, Yahtzee requires a bit a writing and mathematics. Speed is another reason for a computer version. When you consider the time it takes to roll dice, write scores, and add them, you would fall way behind in a race with a computer doing the same thing. This version of Yahtzee does all that for you, and what is better is that you can have competition.
Competition is what makes games more challenging.
So an opponent is included for you to test yourself against. If you're an experience Yahtzee player, don't expect extreme difficulty in defeating Joe, the computer player. He hasn't been playing that long, and needs to learn a few things. A few more things to make computer Yahtzee include a statistical database that keeps track of scoring data for all players. And to ease selecting dice to keep between rolls, computer assistance can make selections for you, to help your chances of getting the score you want.