[slothtrop]

It's the lack of personhood, and low demonstrated consciousness, in spite of sentience. We have a social contract with each other, not animals.

[lo_zamoyski]

Social contract is an awful concept. We have moral duties toward one another, but not because of some hideous contract.

[slothtrop]

We have moral duties to the extent we collectively share values, which are arrived at subjectively and change with time.

In the secular world in the West, even human life in itself isn't considered sacred, as exemplified by sentiment on abortion. The moment we pop out into the world though, we assume personhood and are protected by the social contract.