Can someone explain why electrons cannot exist in orbitals and why we need to describe their location in terms of probabilities.

The Rutherford Atom claimed this theory to be true. Well, as you know, electrons CANNOT exist in orbitals. Why? Well, if the Rutherford atom fit, the electrons moving in their atomic orbits would constantly give off energy in the form of electromagnetic radiation. This energy, according to the first law of thermodynamics, would have to come from somewhere, so as the electrons gave up energy to electromagnetic radiation, they would gradually spiral in toward the nucleus. At this, the electron would fall into the nucleus and the atom would cease to exist in the form we know.

Ah, you sir also need to look up Heisenberg's Uncertainty Principle. Well, you need to so some research but one part of it is that at the quantum scale, any measurement significantly alters the object being measured. So, in other words, if you try to measure something at the quantum scale, your act of measuring it alters the actual measurement.