And this fact is due the pole exclusion principle that states that any orbital has the maximum two electrons and these two electrons have opposite spins. Now notice that electrons can still go into this orbital, right? But they don't want to. And so that's why you won't find electrons there. If the electrons do somehow end up in this orbital, this orbital will create a destabilizing effect. And that means the nuclei will repel one another and will try to break that covalent bond.