You only think they're not a moral equivalent to slavery because you think humans are morally superior to animals and can use them for whatever purpose they like without qualm. Vegans don't agree with that assessment. From a vegan perspective, animals have an inherent right to life and freedom from human caused suffering. And factory farming causes misery, suffering and pain on a truly mind boggling scale. Billions of sentient animals with emotions of their own, living horrible lives until they die painfully and full of fear.

To me, and to other vegans, that is a moral crime, that is on a level of what slavery was.

It's not just a "preference" that's harmless, to eat meat. It's not like "I like chocolate you like vanilla" kind of preference. It's "I'm ok with using sentient beings' lives and suffering for my momentary taste pleasure".