Why is shading particularly important in newly built houses?

In GDR times, there was a saying (regarding the watches produced there at the time): "Ruhla watches are waterproof. Water gets in, but it doesn’t get out." (which, however, was probably unfair to the quality of the watches) This can currently be nicely applied to newly built houses. In energy-saving times, it is desirable (in winter) to use solar radiation through the windows to keep the rooms warm. The heat (sun as well as heating) should not be released again too quickly. The walls are constructed accordingly. As little heat as possible should be released uncontrollably. Of course, the walls keep the heat out longer - but not permanently. That means once the house is heated up, it becomes difficult to cool it down again. Then only nighttime window ventilation helps. Good in the upper floor, where no one can reach the windows. Bad on the ground floor, where the risk of burglary could increase. Therefore, the preventive measures make sense. So the shading.

And now comes the problem with the new houses.

Experienced on Friday. Outside 34 degrees, inside 23 degrees. Baked a cake because guests were coming, so a nice warm kitchen.. Then 10 people sitting in the living room who had already brought extreme heat with them.
And suddenly it was 27-28 degrees in the living room at 5 pm... despite continuous shading by closed shutters.

So the thought comes up to retrofit an air conditioner, but everyone forgets that they consume electricity and not just a little.

Next time we'll have a barbecue, outside in the garage which is located in the north and hopefully cooler.

I still don't quite understand it. An old building should heat up much faster and it then has, for example, 35 degrees right away!? Why is shading more important in new buildings than in old buildings?

Is it really the case that the windows, despite a better U-value, do not block radiation better and the larger window area compared to an old building leads to heating?

Old building definitely fewer windows.
Old building mostly surrounded by trees.

Sunlight for example affects the floor and heats it up.

The detached old single-family house is unbearably hot... (unless it has mass, trees, or few windows) like the good church... it simply has tons of mass. The townhouse has only two sides with windows... often an avenue with trees in front.

The detached old single-family house is unbearably hot... (unless it has mass, trees, or few windows) like the good church... it simply has tons of mass. The townhouse has only two sides with windows... often an avenue with trees in front.

??? Huh, why does a townhouse have only 2 sides???