It's not, because the idiom is "Don't count your chickens before they hatch." The idea is that you're expecting you already have something before it has actually come to pass - in this case, chickens, which could still not hatch because the egg could break beforehand or the chicken could die while still in the egg, or many other scenarios. But there's nothing stopping you from counting the eggs because they are there.
If the idiom were "Don't count your eggs before they are laid", then that's a different matter, because you don't know if you'll have any or not, but you can certainly count your eggs before they have hatched because they are already there.
