The first time I recall noticing the two-dimensialism was at an art museum in Venice. It all seemed ... well, flat.
I still don't understand the phrase: "before those someone understood the art concept of perspective ..." Why should the concept of realism be so elusive?
It has to do with figuring out how to create the illusion of realism. The greeks did it well with their sculpture, so they understood how people looked in real life, but their 2D art was still flat.
We tend to take knowledge for granted. People didn't appear just knowing what we know now. We build off discoveries of others. The same with art. People don't just know how to portray realism. People have to first discover how to do it or even consider it a possibility in the first place.
We see this today. Children don't draw realistically naturally. They draw quite surreal things without even proper concepts of realistic form. They have to learn to draw more accurately from others. It might even be that without other art around them they won't even grab the concept iof proper form.
When it comes to adults, most adults cannot draw realistically. They either didn't learn or they can't comprehend how to do it. There are people who studied for years who still can't grasp tone, colour or drawing hands. I have been an artist my whole life and there was a point where I couldn't understand colour, and I don't even know how I began to understand it, and I still struggle to draw hands because they are such weird structures.
In summary: realism isn't obvious. We only know how to draw realistically because someone discovered the techniques to do so. And we see how realism is obvious by most children and adults still not being able to comprehend it.