Bedrock and java should be the same

How is that possible that java and bedrock are so different, im trying to do a mobspawner and I build it in the ocean (even if that is considered a river), and when I finished it, not even a single mob was spawning, at first I thought I messed up, I informed myself and apparently on bedrock hostile mob don't spawn on the water. So why in java they do? How is that possible that a game so played by gamers, have two different version, and for what? It's just a crazy thing to do. I don't even want to talk about the combat system, the Redstone mechanics, the enchanting, the saturation even the food is different between Java and bedrock, and bedrock is just broken, how is that possible that to build with slabs I placed 4 going downwards?