It is not proper to do so...shall we sacrifice what is an abomination to the Egyptians before their eyes, and will they not stone us (Ex. 8:22)
The Egyptians were extremely careful to avoid hurting animals; according to the Ibn Ezra, they did not eat meat and would not even use any animal products. It is therefore "not proper" when such "humanitarians," who are so filled with pity for four-legged creatures, think nothing about enslaving Jews and throwing their babies into the river. (Imrei Chein)