Technically we do clone ourselves. Every seven years most of the cells in your body have been replaced. Therefore you are a clone of yourself. And this will happen several times over your life.
But I think you want to know why don’t our cells just make new copies of us all the time. This is where it becomes complicated. For humans each individual cell in our body is pretty useless by itself. They all need each other. So without other cells helping cells will die. Scientists can help cells along but even that has limitations. So that is why each cell you drop doesn’t just turn into a new you.
But there species of plant, bacteria, fungus and animals that can just make copies of themselves. The way they do this is different for each one, but the reason they can and we can’t is that during our evolution humans just never needed to copy ourselves, instead we reproduce through sex as it provides more advantages.