probably because reality is a lot more complex than a computer program.

most objects in the real world are just clumps of atoms and don't respond to messages.

you can take a loaf of bread and slice it into 12 separate objects. do that to a C++ object and you get twelve clumps of 1s and 0s that are completely meaningless.

objects are just attempting to model the world. the model is not the world, any more than a painting or photograph or a story is a real world experience. all are just tools that let you think about the real thing.