Because there are many dendritic channels on the martian surface that do not look like they were formed by, for instance, a crack in the crust sandblasted by the martian dust storms. We know what these look like on the Earth, and they do not look like the rounded channels we see on Mars. The martian channels also do not look like what is produced by lava because on Earth these kinds of channels also look very different. Presumably some liquid was involved that is no longer present, and the only candidate liquid is water. Estimates of the number and sizes of craters overlaying some of these channels indicate ages of at least a few billion years.