Not sure of the answer but I will say the unfinished bottoms allowed me to mount them to technic bricks and attach them to the wall with 3D printed mounts. With smooth plates on the bottom you'd need some complicated stand like they have with some of the bigger sets like the Discovery shuttle or Enterpise.

I wouldn't really consider this that big of a deal outside of a case like OPs where they want to hang them from the ceiling. Additionally, if you think about it, nearly every other Lego set is made the same way you just don't normally hang something like Hogwart's Castle or the Saturn V from the ceiling to notice it.

So, they do with some sets but not with others. As I said, it's the thing that bothers me the most about Lego.

I'm assuming it's a holdover from the Studs Not On Top days.
The Stranger Things house is probably the single best counterexample, in that it has an entire house on the bottom of the house.
https://www.lego.com/en-ca/product/the-upside-down-75810