Of course they matter. Background vistas with full geometry, lighting, foliage, tree's, shadowing etc are still extremely taxing, which is why we've rarely seen vistas or backgrounds as dense in racing games, and certainly no other racing game as outright realistic looking with such environments, not even close. There will still also be certain vistas etc that you actually drive to, so there's that to consider too.
Some people are under this false impression that being open world automatically makes your game more computationally taxing or technically impressive than a linear or non open world game. It doesn't. There are countless other factors that need to be considered such as quality of lighting, shadows, complexity of geometry, density of environments, shaders, textures, reflections, physics, volumetrics, post processing, effects, particles and so on and so on. It's why games like Halo 4 or Last of Us, despite being largely linear, are both still more technically impressive and likely more computationally optimised and still demanding than say Sleeping Dog's or Assassin's Creed 3/BF.