They’re a relatively high source of dietary cholesterol. When we first figured out that serum cholesterol is correlated with cardio problems, we basically assumed that dietary cholesterol must be bad. We’ve since figured out that dietary cholesterol is a much less significant driver of serum cholesterol than one would naively assume.
I mean, the Framingham study found no link between dietary cholesterol and serum cholesterol in non-diabetics (e.g. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6024517/). It doesn't really get bigger or higher quality than that.
Oh yes, they had cholesterol (and they still do)... it's just that they CAN be a part of a healthy diet, while breakfast cereal arguably can't no matter how many vitamins you add to it. You'll still find plenty of egg white only things out there in the dairy aisle or at your local diner... since it was the dreaded yolk with all the fat (and vitamins and cholesterol) that was supposed to be bad.
And.... it turns out that fat is totally harmless. Saturated fat, anyway. What we were blaming on regular fat turns out to have been caused by sugar and trans-fat. Trans fat, or hydrogenated fat, was finally (almost!) banned in the US in 2017. (Although you can still buy Crisco!) Trans fat was invented in the 19th century as a way to get people eating cottonseed oil margarine. When fat got blamed for what sugar was doing, they pretended hydrogenated cottonseed oil, and later other hydrogenated oils, were not at fault.
Even people who have learned that sugar is poison often still believe there is something wrong with fat.
Eggs have plenty of cholesterol. But there is nothing wrong with dietary cholesterol. Your liver makes it for you all day long. Blood cholesterol measurements are about markers for circulatory disease that is not caused by cholesterol.
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.
Eggs were ever considered unhealthy?