Not sure if this is a stupid question or not, but from what you say one could only become a designer through first being a programmer/concept artist, but could you do this through other roles in a development team(like say 3D modeler)?
PS. I never actually said anything about wanting to be a designer, it was more that i though it would be a fun and easy job but i was clearly mistaken from the reply’s in this thread.
The problem is that a game designer is broken up into many skilled positions.
EX: Concept artist, 3D artist, Writer and storyboards, GUI, UI, etc.
Where I’ve worked, artists and designers are very separate roles. Level designers sketch out level layouts, grey-box them, test them, then hand them over to artists, who make them look nice without changing the way they play.
In Pure, for example, we used a heightfield with projected textures for most of the landscape, with poly-modelled objects placed on top. The designers provided the initial heightfield, and placed stock objects (courtesy of the artists) such as cuboid buildings and track furniture. They also placed “terrain modifiers”, which were heightfield patches that were blended (e.g. additively) with the underlying terrain - this allowed them to choose from a range of standard features for things like jumps or rhythm sections. So they had access to a lot of the pipeline - enough to create a whole world and run it in the game engine. But on the whole they were nowhere near as good at using the tools as the artists were - they’re different skill sets. The artists came along and ran erosion passes, replaced the grey-box buildings with more interesting ones, textured the terrain properly, populated off-track areas that the designers didn’t care about, placed ground cover, set up the lighting, … and all that jazz. All - in theory - without harming the way the track plays.
So there was a strong distinction between gameplay and aesthetics, and different people hired for each. That’s generally been the case in every commercial project I’ve worked on, though in general I actually think we were rather light on designers - too much focus on aesthetics, not enough on gameplay.
Design work is not just environments though - that’s just the bit I worked on the code for. 
It also includes setting up the vehicle properties, AI tweaking, balancing, configuring career modes, designing frontend flows (usually in close cooperation with a UI artist or three), and a wide range of other game-oriented details. I think it’s fair to say the best artists want to focus on aesthetics, and they probably appreciate having somebody else ensure that the result is functional too. But I can’t speak for them, as I’m a coder.