I've just been playing GTA V again, and one thing that I noticed was just how big and detailed the underground city system was. GTA IIII, San Andreas and IV did have underground train tunnels, but V seems to go even further by having a complete sewer system and a large amount of tunnels and construction sites, like the one you escaped through in the Jewel Store heist. And then you put in the amount of detail in rural areas, underwater reefs and Los Santos itself. Could this be a reason as to why there's less interiors than expected in the game? That Rockstar spent so much time detailing these certain areas that they didn't get around to modelling interior areas.
Still doesn't excuse the lack of restaurants though, I'm many of us wanted to re-enact that scene from Falling Down.