Probably because reality is indefinite complex and that it would take a whole reality to correctly simulate a reality.
So the hard part is to make aproximations that run in REAL TIME (unlike scientific simulations, that have some time to run).
And how do you measure complex? Well, how about the number of changing variables involved?
Simulating and solving chess is very easy in comparison. You have only very limited and fixed variables.
Simulating a physic sandbox game with advanced chemistry and even biology that is supposed to run in real time - seems impossible at this moment on the other hand. Too many connected details.
So the hard part is to make aproximations that run in REAL TIME (unlike scientific simulations, that have some time to run).
And how do you measure complex? Well, how about the number of changing variables involved?
Simulating and solving chess is very easy in comparison. You have only very limited and fixed variables. Simulating a physic sandbox game with advanced chemistry and even biology that is supposed to run in real time - seems impossible at this moment on the other hand. Too many connected details.