Now if i am not mistaken then the unusable tiles are still actually part of the planets surface, still sitting there with the sun shining down and yet nothing can be built there?? Now basically the only thing preventing building there is economic viability which translates to developers being too lazy to develop the land or sea - and that is the basis of planet quality!
Well I think more relevant considerations are game balance, and saturation if you want a RP answer.
The game developers could have added on features just for the hell of it and turned GC2 into a bureaucratic nightmare where every possible sideways expansion into the game is coded in, but if they ever managed to actually release the game would it be fun? Probably not.
I was'nt sure if when you described the developers as lazy your talking about the game developers or hypothetical planet developers. If you did mean the game developers then go code your own damn game, much less actually try and earn a living for your household doing so. These guys have done a great job and made it possible for us to mode the game ourselves whenever we want.
If you meant the imaginary planet developers running around on Kora or wherever, then the PQ thing answers that. If the planet is low quality then it has mountain chains, chasms, swamps, tar-pits. And yes after having researched the teraforming tech you can dry out that swamp, but you still can't build your factory on the side of that mountain, it will fall off.