Notably, the great majority of them don't have the slightest clue about farming in general or factory farming in particular. Don't mistake social signaling for actual positions.
As the expression about knowing "how the sausage is made" attests, generally the more people learn about it, the less they like it.
Of course, veganism is very far from being an immediate consequence of disliking factory farming. (Similarly, refusing to pay taxes is very far from being an immediate consequence of disliking government policy.)