Yes, most corner positions are as advantageous as Darka's, which I think is a shame. It's a nice surprise to see situations where the powerful realms are located in the centre of the map, because I know that in the future those realms will probably be torn apart.
Except that historically, in BattleMaster, the highly influential, imperialistic realms have tended to be in the center. I'm looking particularly at CE and Enweil here.
Darka is, indeed, reasonably well protected—but, as several others have said, they have never tried to build an empire. They just want to hold onto the land that they have, and be hired to fight other realms.
So your assertion doesn't really hold up terribly well in practice.
If I were a map maker, I would locate the richest and most traversable lands in the centre of the map, while restricting the corners to relative poverty and long travel times. This would balance out the inherent benefits of corner realms with the inherent disabilities of central realms.
Well, y'see, part of the problem here is...none of BM's maps were really designed to maximise strategy...or promote certain ways of playing or thinking...or really "designed" much at all.
But you're also forgetting one other very important thing: Soon, sea travel will be coming to Atamara, and then no one will have a completely safe sea border.