Because they want to? Generally speaking vegans are vegan for the animals, whereas plant based people do it for their health. Why do people eat sweets, crisps, red meat, etc.?
Veggies and vegans go for “fake” meats because, as I said, they’re in it for the animals not because they dislike the taste of meat. It’s also an easy thing to shove in the oven if you’re feeling lazy, like chips or a pizza.
Do you only eat nutrient dense, healthy food?