Awesome, Kowz is talking my language.
I don't believe the inhabitants of DK Island would have gone to the trouble to do excavation/landscaping just to create those long stretches of dirt... Sure there may be a few beaten paths, but they'd bend around trees and rocky outcrops, and some areas of foliage would be way too dense. It's just presented that way in the game because of the art and design that was appropriate for the genre. I'm sure they'd be curving every which way, especially to find those hidden bonus areas.
I've thought of a couple of counter-arguments, though:
Embankments; DK and Diddy have to jump quite a bit, everywhere they go... if the path they followed was the type to go around obstacles, surely they wouldn't need to vault over so many embankments.
Barrel Cannons; Many of them are probably put in place to get over obstacles that could be walked around, if the Kongs decided to do it that way.
Bonus Areas; lots of bonus areas are like shortcuts directly through rockfaces. Again, moving straight through instead of around.
Kremling escape route; the Kremlings would have tried to get away from DK as quickly as possible as they stole DK's bananas, and so would have probably chosen the most direct path.
That last one might hold some clues, though... Didn't the Kremlings take DK's bananas away in carts of some kind? They would have to take them on cart-ready terrain, for the most part... I do wonder how they spilled so many in mid-air around Barrel Cannon Canyon, etc... (and why there were so many left floating underwater)
Eh. I'm thinking about this more than is healthy. Better head off to bed I suppose!