Why bother with the tournament?

I don't recall this being clearly explained in the movie, but Wikipedia says:

Once every generation, there is an inter-dimensional martial arts tournament known as Mortal Kombat, designed by the Elder Gods to limit invasions between the realms of the universe. If the realm of Outworld wins Mortal Kombat ten consecutive times, its Emperor Shao Kahn will be able to invade and conquer the realm containing the Earth.

The movie does state that if Shao Kahn's warriors win one more time, he can lay claim to Earth's realm. Shao Kahn and his warriors are shown to be extremely capable, and could likely take over Earth's Realm with a large enough show of force.

Why does Shao Kahn need to abide by this "ten win" limitation before he and his forces can conquer Earth? How are the realms protected from them just skipping the tournament and invading?

In the absence of a movie-universe answer, a game-universe answer would be acceptable here.