Why is shading particularly important in newly built houses?

That’s how I did it too - but my kids don’t have to sweat and my office is in the basement, where you don’t sweat anyway.

I also find it a bit selfish to cool one's own bedroom and let the child/children's rooms, where perhaps a PC etc. is still running, heat up. Always blaming the little ones

Steffen has basically everything included.

The U-value, as others have already written, is not decisive for solar gains. The U-value only refers to thermal conductivity: it allows a very good estimation of how much energy flows when there are different temperatures on two sides. Radiation is not captured by this.

Furthermore, houses today (firstly) mostly have larger windows and (secondly) less massive masonry that stores heat. Keyword: phase shift.

We are installing a climate control unit in the attic that cools the children's rooms and the bedroom. Doesn't cost the world, maybe 3,500 euros.

Another argument is certainly that new buildings, compared to "older" buildings, usually also have significantly more window area. This means that, without shading, considerably more heat naturally enters the building through the windows...

Everyone here with their new buildings is experiencing this right now... whoever didn’t have the shutters down, just suffers...

Hello,
that’s nonsense! It depends on all parameters: window area, house orientation, room layout, overhangs, etc...

We build without these ugly standard shutters because other parameters were taken into account!
(House orientation, a lot of KS mass, roof overhangs, room layouts, and the use of sliding slats)

No, it’s not nonsense – this applies to almost every house, because the absolute majority will open towards the west/south and then it gets hot. Or is your house only facing north and invisible on the other sides?

And don’t take the word "Rollladen" too literally, but replace it with "external shading element."

The details certainly depend on nuances, but in the end, it probably makes about half a degree difference.

By the way, I only have KS blocks, paid a lot of attention to summer heat protection on the roof, have roof overhangs, and we have no glass front facing west/south.

Oh, and ugly external shading elements on the upper floor – absolutely terrible.

And yet it still gets warm in the place.