Because vegans lack common sense, and have no understanding of the natural world.
Their existence is only possible in a post-scarcity Western society, where there are lots of vegan alternatives which are only available by raping and bankrupting developing nations for their avocado toast, cutting down rainforest to grow their monoculture soybeans, and destroying and polluting the environment for their cotton clothing and ocean-destroying plastic ‘leather’.
They need to spend a year or two living on locally-sourced turnips and wearing nettles to knock some damned sense into them.
Quora linky.