There literally are organised boycotts of the animal ag industry. There are local campaigns to switch to vegan catering at offices, universities, etc that gain traction and sometimes succeed in making policy. The average westerner consumes huge amounts of animal products, over the course of e.g. a year it does add up to a significant amount of money not given to the animal ag industry especially with the volume of people that are vegan.

I think the main reason though is to prefigure a communistic mode of food production and consumption though where we are not at odds with nature. You could argue that we shouldn't feed the homeless because they'll go hungry anyway even if we give them one meal, but should we not prefigure a mode of existence where we care whether our neighbours live or die? Should we not try? The animal ag industry is completely unsustainable—we are not talking about hunter-gatherer peoples hunting, or foraging eggs, or whatever. We are talking about capitalism cramming as many chickens as possible into a cage with a floor area the size of a iPad, stuffing animals full of growth hormones, destroying entire ecosystems just to meet western beef demand. Because the average global southerner doesn't eat that much meat anyway—it's an inherently expensive form of food as meat has to eat plants, so instead of growing plants directly for human consumption we have to grow plants to feed animals who burn most of the plant energy instead of giving it to us when we eat them. If we are to envision a communistic mode of food production, where food is produced to best meet human needs, and we don't ceaselessly produce for the sake of profit, it is at the very least going to be far more vegan than we currently are. Why not try practising it?

There are plenty of problems with liberalism in the vegan movement, but as communists it is our job to politicise, to radicalise, and to make militants out of activists and workers. Veganism isn't completely wrong. Its foundation is by and large correct, and it can easily be steered into a communist political direction.