Why do pleated blinds have to be 2 centimeters wider than the windows they are covering

In terms of vertical load coming down, there would be little load under the first floor windows as they always have a header above the window to spread the load to jack studs on the side of the windows. In addition, there are full height king studs next to the jack studs that extend to the top plates.

There may be cripple studs under the window to help reduce any small sagging of the window frame. Your steel angle bar may be needed to support this and the floor joists above your basement windows, depending on how much vertical dimension you want to allocate to this.

If I were doing this, I would determine the length of the floor joists and calculate the square footage supported by the joists above a window. Then use the code required load (typically 40 pounds per square foot for residences) to determine what your basement window header must support.

This link shows how to calculate load: loads-on-headers

And this link shows the beam size required for a certain load: load table

They're not to everyone's taste. They roll up into a large box above the window -- either inside or outside. And they are more expensive than blinds. Maybe Europeans like them for the noise and privacy factor and that may not be as much of a concern here in the states. Having said that I have wished many times that I had them in my house. Favorite feature is that you can open them up just a little bit so that it has holes in it and you get some light and some breeze. But also very nice to just shut out the world outside and get a good nap in the middle of the afternoon!