Because horses are very useful on plains?

I think they are trying to match races to a terrain that plays to their strengths. Humans' strength is mostly in numbers, so a vast levelled area is good for human troops to wage war (far better than a craggy hills where a single tough but not mobile dwarf can stop a lot of enemies). Lizards are used to heat so dragonborn dominate their enemies in hot deserts of the south. I'm sure some of them live in mountains or plains but those are not the places they have an advantage. Halflings are not warrior race and have to resort to either fleeing (rivers) or smarts. They are small and light so swamps make an ideal hidding place because every bigger enemy (and most of them are) will be simply sucked in.
Who said there isn't an occasional dwarven city in a forest or a elven town in a swamp? This is
generic infomation about the setting and is by no means exclusive.