It is difficult to organize non-trival projects in AD without pages. For example, I have a website with 40 views, each with its own layers showing the base view, notes, actions, and datamodel.
But in addition to these basic layers, every page/view has custom layers to manage overlays, popovers, sliding panels, menus, special action behaviors, or in complex views multiple data-models. If I had to flatten all layers for 1 master artboard view, I'd have 100s of layers to manage, and it wouldn't be clear which artboard view they go with. In addition, managing shared layouts with shared layers, is a very helpful concept.
Artboards are not practical solution for non-trival design projects. Artboards are nice for managing aspect ratio variants. Both pages and artboards are used to good effect in Sketch, which with omnigraffle problems own most of the UI/UX market.