I think some vegans are doing it for reasons which cause them to believe restriction is the key to a good diet. They restrict meat, and then dairy, and eggs, and honey, and leather, and pearls, and silk, and palm oil... and sometimes they keep looking for more things to restrict, like processing, packaging, oil, cooking, and so on.
I am vegan for the animals. To me it isn't a diet but a way of life. Cutting out oil would be terrible for my health. Maybe other people can survive with very little oil, but not this vegan!