Good looking terrain doesn't require a high density mesh. Low poly terrain can look natural and this is thanks to good lighting and textures. When 3D graphics were in the beginning, before texturing was even possible, terrain would be limited to prisms and pyramids to represent mountains. That's no longer the case. Hiding the triangle's edges with properly aligned textures and smooth lighting does the trick.

I have no idea why the mountains were made with giant flat surfaces like that. In the same game there are canyon walls with good modelling techniques. I can only speculate that either this level was rushed or that whoever made the level didn't care about good looking terrain. Technology can't be blamed or excused because the same engine was able to produce other levels with much better looking geometry.