Simple!! because we all care about non violence, Justice and Freedom.
Whether we love animals or not, we all believe that it is wrong to inflict unnecessary suffering and death on animals. For example: You would consider it wrong if someone burned a dog just for the sake of fun. Which means you think that it is wrong to cause suffering to the dog when it was absolutely unnecessary. But 99.9% of all animal use is either for pleasure (taste), amusement(fun) or convenience; which cannot be considered a necessity.
We cannot justify it on the ground that we need to eat animal products for reasons of health. We clearly do not need to do so. In fact, the evidence increasingly shows that animal products are detrimental to human health in the long run.
The only justification we have for inflicting suffering and death on 53 billion animals every year is that we get pleasure from eating/using them; that it is convenient for us to eat/use them; that it is a habit.
In other words, we have no good justification at all. And yet we all take part and support this tremendous unnecessary violence that results in the deliberate slaughtering of 53 billion animals every year.