Pretty graphics put more load on the CPU. All graphics are passed by the CPU to the GPU. This takes up more cycles. These cycles that would be used for flight modelling, damage modelling, or ballistics modelling. All of these put high demand on the CPU.

That's why most of the games with great eye candy have limited physics modelling.

With flight sims there's a trade off, in the next few years that trade off may become very little, but right now it's still fairly significant.