The reason a game model can't be 50k polys (yet) is because in a game everything has to be rendered in real time.
You would have to have some uber sweet Game engine to do it... (which we will have some time soon-ish)
Could you imagine if a modern day game engine had to render 50k poly meshes... Not to mention if that were for only one character... then image the hurt the engine would be feeling trying to render 4 or 5 characters to the screen....

^^^^ ______flat line______
Not to mention the environment..

So, It's all because the game engine has to render all the scene objects to the screen as close to 'exactly as it happens' that in can.

At least this is what I know... There could be other dark sinister reasons...