The answer is because these are games. All games that I can think of, with the exception of Chess, each have a certain amount of luck designed into them. Its what makes them fun and its what makes them games. Some games, like the Olympics, have more skill than luck designed into them. Others, like casino style games, have more luck than skill designed into them, but superior skill or dumb luck can bring you a win in both types at times. Removing luck makes it just a mathematics problem that you can do on paper. No need to program a computer simulation to prove what you already know the outcome of.
Once, a long time ago, a Roman was bored. There were dead legionaries everywhere, as there always was at the time, so he stole some knucklebones while stealing a pair of boots.
He was arrested for stealing the boots, and had nothing else to do but roll the bones on the ground with his fellow prisoners, who were quite comfortable with this new passtime as it gave them something else to do other than tame the rats and lice and make them do tricks, by this time old hat.
It is a well known fact that rats liked to hoard things, and so it was that the rats had stored an immense amount of Greek silver. When the rats gave this silver to the prisoners, the only logical course was to buy the prison they were in and open a casino. Thus gambling was born.
Meanwhile, a Chinese noble had taken the marbles from his head. Unlike modern practice, where they are placed in a small bag connected just below the belt, the Chinese noble decided to use them to teach his son patience, who was killing peasants to pass the time. Thus, board games were born.
Very soon after someone suggested combining the random elements of knucklebones with a board game, adding enough mathematical record keeping to make the game as protected as alchemy and suggesting a premise of futuristic star-ship combat. Of course, this man was quickly burned as a heretic, witch and all 'round crazy nut, but it was little known at the time that this form of game would eventually re-emerge.