People have complained about the lack of many interiors in the game. Well most likely the large countryside/wilderness areas are largely to blame (either space or time issues, or both). So that begs the question, would you have been fine with this game being simply the city of Los Santos, with little to no wilderness surrounding it, if it meant being able to enter a lot more buildings?
Personally, I'm glad they did it the way they did. Interiors only exist when you enter them, but the entire world exists wherever you go. More interiors can probably easily added by DLC if they wanted to; the same probably can't be said about all of the countryside/wilderness. Also I'd take fewer, high quality interiors, over a larger amount of them that seem useless (felt like half the interiors in GTA IV were just apartment buildings that were only used for missions). Really the lack of being able to enter Burger Shots and the casino are my biggest complaints, but again, those things could likely be added as DLC.