Do we know that's even the case? I mean, if Pica is like a stone Logia, and Zoro hasn't actually shown yet that he knows Armament Haki. If that's the case, then there IS no place he can cut Pica.
Devil's Fruit users AREN'T human.
If using your fruit ability consumed your stamina, Sugar wouldn't have been able to keep her power constantly active for 10 years.
I would think that any character without Observation Haki would be vulnerable.
I mean, it worked on King Riku, and King Riku is actually somewhat strong going by his performance in the Colosseum.