In general, when modelling reality (either for games or offline simulation/rendering) developers are focused on techniques that have a significant impact on the

In general, when modelling reality (either for games or offline simulation/rendering) developers are focused on techniques that have a significant impact on the perceived realism of the result while also offering the possibility of an efficient implementation. So PBR is widely adopted because it increases realism for little additional cost. More advanced grass simulation would of course be possible but the cost/benefit return is debatable. Particle systems are widely used because particles are a useful abstraction for simulating all kinds of materials and because they can be efficiently solved using vector processing techniques and GPUs.