You can have whatever you fancy for breakfast. Some people eat a plateful of fried meat and eggs and call that breakfast, so why not have cheese on toast?
I can see why cereal or toast became popular for breakfast, it's a cheap meal. Some people couldn't afford 3 full balanced meals a day with meat and vegetables, so they focus on providing a proper dinner and the other meals are just filler really, or they just want something light at that time of day.
Anyway, I've just had a cheese and onion chutney sandwich with salad on the side. More of a lunch really, but I don't care.