I used to think the same. I would guess that you do not have a big breakfast. Without getting a real meal in for breakfast, hitting a huge calorie surplus is difficult. If you counted your calories and tried to get a 1000 calorie meal for breakfast, hit 3000 calories a day, you’d probably gain 10% in a few weeks. Weight training is good too… you don’t want to just gain fat.
1000 calorie breakfast = bagel with cream cheese, 3 eggs, banana, some berries, protein shake. It’s a whole lot more than a bowl of cereal.