Well, it's less about the total amount of calories as it is about the amount per volume of food. 300kcal of egg whites is just a lot more food in size, and some people (myself included) just enjoy eating larger quantities and are willing to sacrifice some quality in taste. Also, if you're paying attention to protein intake then 300kcal of egg whites is also a lot more protein than the equivalent amount of whole eggs.

The fat isn't inherently bad, but there are tradeoffs that some people prefer.