Why is almost all the landmass on one side of the globe?
NVZ is correct. Get a palaeo-geographical atlas and see how the the distribution of the continents has changed over time. There is much more ocean than land, and it has ever been so, but there is no particular pattern to the way the continents have drifted. It's not quite random because the heat distribution in the mantle isn't random, but given the conveyor belt system of plate tectonics there will always be a tendency for crustal plates to clump together before breaking up and then clumping together somewhere else.