First thing is they are hardware accelerated 3D graphics engines. Firefly is doing all the math using your general purpose CPU, not a highly tailored GPU. Also, there are lots of things a GPU cannot do at all - things that are left out because they would be too slow even for special hardware. Raytracing, for example. Real reflections and refractions of arbitrary content arranged in arbitrary ways is just not done.

Second is that game content is highly optimized to take advantage of every possible cheat, particularly knowing that the scene is fixed. For example, most shadows in a game are pre-computed and baked into the scene. The sun doesn't move so the shadows don't have to move. Reflections are generally fixed, since the scenery is fixed. In a car game when you load a track, all the shadows and reflections are baked into the textures.

There are other reasons, but they all boil down to speed versus realism and flexibility.

If you want to render in real time, get a gaming engine, convert your content to run in it, and then you can make movies in real time the way games do. Google "machinima".

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)