Cheap/easily available, tasty/desirable thanks to sugar, trivial&quick to make and eat when time is at a premium - that sounds perfect in general, doubly perfect when you're trying to ensure kids eat something relatively nutritious before school. I imagine that's why cereals stuck around - it's the "trick kids into drinking milk" plus energy booster for school children.
(Also, sure, it's "better to take the time to eat a good non-processed food breakfast", but most people have neither the time nor money for that.)
Unlike breakfast cereals, they do not contain sugars, but when eaten simultaneously with sweetened milk, there will be no noticeable taste difference. When you sweeten the milk yourself, the amount of added sugar is normally far smaller than the unbelievably high amounts typical for breakfast cereals.
Then I have made a very detailed analysis of everything that I was eating and I have begun to weigh myself every day at the same hour and in the same conditions with high resolution scales, to be able to assess the effect of the changes in my diet.
After replacing the junk food that I was buying with food cooked at home from raw ingredients, after a year I have reduced my weight to two thirds of the initial weight and then I have kept it constant for the next two decades.
The worst offenders among the junk food that I had to completely eliminate from my daily intake, had been fruit juices and fruit yogurts and breakfast cereals, all of which contain excessive amounts of sugars, regardless of producer.
Example: https://www.uncletobys.com.au/products/cereals/uncle-tobys-v...
I had a conversation with my wife a few years back, about how a lot of things that are deemed acceptable or situationally appropriate are cultural. And sugared-breakfast-cereal vs real food came up.
My conclusion was, cereal is cheap and sugar is addictive. Better to take the time to eat a good non-processed-food breakfast.