This is a very niche reason, but one of the classic diets for beginner weight lifters who want to gain weight is to add a gallon of whole milk a day to their existing diet.

For people that are trying to consume more calories and protein, I don't know of an alternative that combines comparable calorie & protein density, low cost, ease of consumption, and zero preparation time.

More generally, I think that one of the big challenges with plant based diets is a lack of easily available calorie and protein dense foods. (I'm not saying its impossible to get enough calories and protein as a vegan, even as a bodybuilder. Just that it is not nearly as easy.)