You can buy 5' longboards. They're uncommon. I find the build quality on Kahuna to be lacking.
My understanding is that the tiny wheels keep you closer to the ground while also allowing you to turn tight, which is necessary for tricks, which is something longboards don't do, which is why some people put sidewinders on longboards, which requires a lot better balance than I'll ever have.
People have been known to do dumb things to get lower.