Same reason we use electricity despite not being 100% green energy and thus being even worse for the earth?
If you actually wanna guilt this question then the fuck are you doing using your coal and gas powered electricity to do it?
There is no ethical consumption under capitalism, because the capitalists have seen to it that you will never be permitted to make an ethical choice that would dare compete with what they expect you to choose.
Being a moralizing prick doesn't send any message, what gets people to change is making that change easy, that's why instead of being terminally online fuckwads, british vegangelists spread the good news by hosting free kitchens, volunteering to take people grocery shopping on their own pound, teaching vegan cooking classes, and all other sorts of actually addressing literally any of the actual concerns people have about going vegan instead of being a condescending snob about it.