It is said of everything from outdated 3D games to the best of the 2D era to text-based MUDs and Rogue-like adventures. You know how it goes: ‘Man, that game was powered by the best graphics engine in the world: imagination.’
Sure, you can imagine a fictional world in greater detail than that in which it can be presented to you. At what cost? A couple of detriments seem, to me, to weigh quite heavily against the argument:
Imagination is of considerably lesser bandwidth. Although high-quality, it is slow. It takes quite noticeably longer to imagine a scene than to take it in visually.
Also, with regards to the world creator's intent, your imagination is less reliable, less likely to accurately represent the intended visuals. One might argue that this ‘personal interpretation’ is the beauty of seeing a constructed world through imagination, but hell, we can all imagine anything at any time as we please; don't we enter the worlds of others to experience their imagination?