Ich is most likely very common in the wild, however, due to the size of the ocean, it doesn't concentrate as it reproduces, allowing much of the new 'ich' to die off without a host. Additionally, there are stronger currents, that carry away the ich once it falls off the fish (sweeping it away from the reef) and even if the fish didn't move, still gets them away.
Even additionally, the concentration of animals that can eat/cleanse parasites from fish is most likely much much greater, also putting some impact on the ability to control the parasites.