What is the biologic incentive for a homo sapien to give up 100% of all animal based foods based upon the pain and suffering of some animals used for food?
As to "why have empathy for non-human animals?": I don't know, but I guess having a somewhat-arbitrary rule of which animals it's ok to eat, and which not, can lead to a certain amount of congnitive dissonance.
There are also health issues with meat, that don't exist (or don't exist to the same extent) with a vegetarian diet.
That's just it it isn't a biological incentive, it's a moral one.