For liquid water to exist on the surface, a planet needs to be in a certain zone not too far and not too close to its sun (among a few other factors). How many planets per solar system do you think can fit in that range? Maybe three max? I don't know, but the point I'm making is that we only have a pool of 8 to look at. Sure we can see planets in other solar systems, but we can only see them when they pass between their sun and us. We can't observe well enough to see water. It's like pulling 8 kids out of a classroom and finding only one that is black and concluding that he/she is the only black child in the world. The sample size simply isn't big enough. For another example, Mars is the only known planet to have two moons. Does this make mars special? Does this mean that there aren't any other planets in the entire cosmos with only two moons because it's the only one we know of? Does this mean that Mars is special and because it's special God must have made it? No.