* Colors other than gray don't look nice in CAD and aren't the default.
* It's super easy to just stack a bunch of rectangles (looking at you, architects...)
* You can chamfer and fillet but that's the end of what most people do. More complex shapes are hard to due due to the clunky spline tools. Hence elaborate ornaments are left out.
If your building is made from rectangles, lines, and few curves, you can use off the shelf panels to build it. Order them from a factory, have a truck deliver them, stick them together with cement and bolts.
On the other hand, even if you do pay someone for the extra time to create beautiful, unique, complex designs in a CAD program (which is not that hard), actually making those parts is extremely expensive so they will get rejected by most architectural and construction firms.
Perhaps as we come into the era of 3d printing, more complex structures will come back into vogue again. It will be kind of ironic if 99% of all futuristic cities depicted in scifi, all clean lines, spires, and cylinders turns out to be wrong and the skyscraper of the future looks more like the Sagrada Familia, but built in six months instead of three centuries.
(Although personally I'm hoping the future looks like forested garden cities.)
in the era of blueprints, where you had to draw literally everything from scratch every time, human pride would necessitate taking the time to add flourishes or small details to every page, just to keep the talent who is producing them from losing their minds
look at old bibles, full of small details that are ultimately useless for the message being conveyed, but still announce the presence of a poor soul doomed to reproduce these glyphs for eternity
nowadays... you can just open an old drawing, hit copy-paste, and be down by the pub at 11am.... not a bad deal for the drawer, but for the people looking up on the sidewalk...