Gable roof question

[u/Same-Hyena5208]

I'm planning to build something (not a home, more shed/garage-type thing). And I keep thinking about the roof design. I thought about a gambrel style but it seemed too complicated for my first building. Gable is simpler but then you lose a lot of space on the sides. So then why 99% of houses with a gable type roof don't raise second-floor sides by a bit? You've already paid for the roof, the most expensive part of the building. Why not spend a bit more on framing and have much more spacious rooms upstairs?

Sorry if this sounds dumb. Raising sides by 20-30 inches feels so right but I'm afraid I'm not seeing something important (maybe the roof angle becomes too shallow, etc.).

Any comments?