That's because it's a projection into two dimensional space.
Because we see in 2d, and only think in 3d.
Well we don't know that, it's only a hypothesis. The idea is that because they're curled up into themselves and so only manifest at extremely small scales. Kinda like when you're looking at a piece of strung wire from far away, it appears to be only a straight line, a single dimension. But for an ant it's a three dimensional object that it can crawl on and around.
Ant and the wire crawling along it... has every seen that science video? Anyway, taking advantage of these 4D pockets would be like taking advantage of a wormhole. You could use them to bypass anything along the first 3 dimensions. We can't do it yet, but the wormhole thing we're already getting towards.
There's that too. Why would an inanimate cube be the body of a omnipotent being? 2D objects are projected to us in 3D, we just see them as "flat." That doesn't make them capable of doing what 3D objects can.
If there are more dimensional planes, then we are probably moving along them. We just can't fully perceive this process, and therefore have to sit by and let it happen, until someone gets lucky and finds the thing that will allow us to do so.
We are stuck moving linearly because we lack the locomotion. We probably exist along more dimensions than we see. If we did create a means we could bypass all conventional physics and be gods. *cough cough* Which may have already been done.*cough cough*