Because it's easier to harvest free oxygen on land. So since we have land, free-oxygen harvesters live there. Then they turn around and eat anything close to

Because it's easier to harvest free oxygen on land. So since we have land, free-oxygen harvesters live there. Then they turn around and eat anything close to the surface that could evolve into a land-equivalent. But on an all-water planet, the putative bio-land makers would not have to compete with actual land-dwelling creatures because there wouldn't be any. [UPDATE] Just to be clear: there's obviously no way to know whether this would happen. I'm only arguing that it could happen, and so lack of land is not necessarily a show-stopper for industrial civilization to emerge.