The why is not really known. But we simply cannot. There is not line where one particle ends and another particle begins. The best you can do is give a probability distribution, but some of the particles will be in places where they’re not really supposed to be. This is actually what drives the fusion processes in stars. The nuclei don’t actually have enough kinetic energy to fuse–but she is the protons in one hydrogen nucleus just magically appear in the nucleus of a neighboring hydrogen atom.

You literally can’t have distances that are smaller than these probability distributions.