For example on wikipedia for Switzerland it says the country has an area of 41,285 km². Does this take into account that a lot of that area is actually angled at a steep inclination, thus the actual surface area is in effect larger than what you would expect when looking onto a map in satellite view?
In addition to the other reasons people have given for why they don’t: we’re usually less interested in the surface area of a plot of land than we are in the usable area.
People generally can’t live or farm on a significant slant and will instead level the ground or build supporting structures to make things level.
Things like rainfall are also generally better calculated by flat area.
But here we have Switzerland as the example :) where nearly every small or large piece of farmland is far from level.
“significant” being the keyword. :)
There’s clearly a slant, but you can see where it was a bit too much and so they added retaining walls to level things out.
In a place like this, you can tell what places are suitable for farming, which could be farmed with a little leveling, which are suitable for grazing, and which are just too steep for food.
They’re definitely not going to entirely level every place, but you also can’t grow food on the side of a mountain. :)
The photo of the terraced farming actually brings up an interesting point–in order to render those slopes usable for farming, terracing approximates the “flat” projection of the terrain anyways, so you end up with the same result. Buildings and any other usable structures follow the same rule: you can only build vertically, so the effective surface area is the same as the flat projection.