Fish is one of the least calorie dense meats. Vegetables and fruits are some of the least calorie dense foods. It's nearly impossible to eat enough lettuce in a day to go over your daily recommended calories. It just won't fit.
So, in a lot of ways, keto, paleo, vegan, Mediterranean diets are all sneaky ways to cut out a major source of calories. You don't have to count them because just the way you're eating is taking care of that aspect.
But if you want to eat Twinkies exclusively, you can do that as long as you eat only so many.