You are assuming a linear behaviour for human decisions, but in the same way that a person would eat a cow but not a horse, some people draw the line at fish, others at eggs, and so on.

As a vegetarian myself, I have read that the economic, health and environmental benefits of not eating meat draw a curve with an optimal point right before becoming full vegetarian; going vegan can easily feel like going too far since the price of ingredients and carbon and water footprints start going up again when you start to diversify your diet to replace the things you liked before and that is before considering the social impact of not sharing other people's food.