I hate when Lego doesn't do anything on the bottoms of sets, this is especially clear on Star Wars spaceships.
(media.piefed.social)
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