While what you describe is certainly the ideal, and likely applicable in a variety of situations, there are a lot of real world situations where it can't cut it. Antirez says it best:

   remember all those stories about DB denormalisation, and hordes of memcached or Redis farms to cache stuff, and things like that? The reality is that fancy queries are an awesome SQL capability (so incredible that it was hard for all us to escape this warm and comfortable paradigm), but not at scale.

You'll be hard pressed to find any medium-sized project in the wild that doesn't require a layer of denormalization or caching to be reasonably responsive (I mean, some frameworks come with that built-in - their users might not even be aware this is happening). You might have some beautifully crafted model underlying that layer, but don't fool yourself into thinking that's all there is.