Because in most cases landing in the water is no different from landing on concrete. In fact, it could be much worse.
I'm not going to talk about the technicalities and physics behind it, but there are loads of reasons why you shouldn't land a plane in the water.
For example, the waves are a big threat.. Pilots usually try to land parallel to the waves, so the aircraft isn't pushed around and endangered. In the scenario that there are waves directly moving towards the aircraft, it's like running into a wall that's moving towards you.
And the most worst case scenario comes to the aircraft breaking apart. If an aircraft breaks apart upon landing, it greatly endangers the passengers and crew as the aircraft will begin to flood with water and sink.
All in all, when landing in the water, there are a lot of extra variables that pilots must deal with the execute a safe landing. So if you were given a choice of either landing on water or land, try landing on land first. Landing on water is always a last resort.