And yet most houses are taken from the catalog, for example the aforementioned Flair 131. Why is that?
For lack of better knowledge and imagination, the tendency not to spend money on the expensive architect, the tendency towards more prefabricated house construction, and the resulting absence of a "free" architect – take your pick.
The Flair certainly has a very functional floor plan in itself, which, when considered alone, perhaps has a larger overlap with suitable plots of land matching the requirements of the builders. But just because it often fits reasonably well does not mean it is the best floor plan or even the most beautiful.
I even believe that many builders who have built a Flair could have had a floor plan that was more optimal/more beautiful/more individual for themselves; it simply didn’t come about due to the reasons mentioned in the first paragraph.