Earlier quoted context omitted.
Because they don't just like it like animals like stuff. They like it as meat, but they also like the parts of their culture associated with it. The same way an American would like the turkey AND the thanksgiving dinner, and would like to continue having both, and not just because of turkey meat, but as a combo.
So would slavery similarly be more justifiable if it was also done not only because slave owners liked it but because it was part of American culture?
The fact that people have now changed their minds on that particular matter doesn't mean that anything people haven't changed their minds but consider tradition is equally wrong.
Or that there's some law-of-nature-style "right and wrong" aside what a culture deems as such. For all our modern posturing, if we were merely born 200 years ago in the US, heck, not even the South, we would statistically be far more likely to be in favor than against the practice (and still be considered perfectly "modern" for those times).