I am vegan. Why am I still hungry?

You have to bear in mind that plant food is less dense in nutrients than animal food. So you have to eat a lot more bulk. Many people simply don't have the stomach capacity to comfortably eat enough plant food in three meals a day to get sufficient nutrition. Try eating five or six meals. Another way is to fill your pockets with nuts, dried fruit, etc., and nibble it all day long between meals.