Because eating grain based food degrades the soil, releases tonnes of carbon into the atmosphere, causes soil erosion (we only have a few decades of harvests left in the topsoil), kills multitudes more animals and wildlife indirectly than directly consuming an animal (earthworms, field mice, rabbits, butterflies, deer, rodent control around granaries), requires huge quantities of fossil fuel based fertiliser.
Whereas animal grazed grasslands are wildlife rich havens. The grazing animals fertilise the soil with their dung. Pastoral land and soil actually absorbs carbon dioxide out of the atmosphere, fossil fuel inputs not required. When you support the soil and nurture the soil, you support myriad more lifeforms. Wheat, soya, corn are monocultures where nothing but the crop survives... vast amounts of -cides are required to maintain the crop growth. -cide = death.
www.theguardian.com/commentisfree/2018/aug/25/veganism-intensively-farmed-meat-dairy-soya-maize
Because meat and animal products contain fat based vitamins like vitamin A, D3, K2 that are not found at all in the plant kingdom. Many people are poor converters of beta-carotene to Vit A. Ditto for most of the minerals... heme iron in meat is much more absorbable than the plant form.
Because plants don't want to be eaten either, like any organism. Animals evade being eaten by locomotion; plants use chemistry. Many plants contain chemicals which are anti-nutrients (eg phytic & oxalic acid), endocrine & immune disruptors (eg lectins & saponins). Soy and wheat are notorious for causing thyroid disruption.
Because it's impossible to eat a vegan locavore diet with minimal food miles in the UK... imports of food from the other side of the world by polluting ships and planes are required... avocados, rice, almonds, palm oil, sugar cane, bananas etc etc. All cash crops, with ecological problems of their own. Almonds are sucking California dry and contributing to bush fires. Rice from paddy fields is a huge cause of methane emissions. Palm oil is decimating orangutans. Just give me some local butter, smidge of honey, lamb chop and cheddar and a few bits of veg.