Why does every zone have mountains

Every one, with the exception of netherstorm, has mountains either enclosing the whole of the area (almost all zones) or at least on a prominent part on the map.

Why does Blizzard feel it necessary to bottleneck everyone though a few small entrances or exits instead of just having a gradual entry into another zone? Granted on some zones it makes sense (burning steppes, searing gorge), but on others makes no sense at all (mulgore?!?).

I'd just like to see a more fluid transition from zone to zone in WotLK i guess.

Minor quip