It's not just about the number of sensors. The clouds form over the sea. Predicting when and where those clouds will form is much harder than predicting when and where those clouds will move.
One needs to account for relative humidity, which the ocean provides. Much of the US landmass between the pacific crestline and the rocky mountains is arid desert (from the canadian border to mexico). Second, there exist ocean-specific dynamics like el-nino, which don't really have an overland anolog.
When warm wet air meets cool air you get clouds. The goal is being able to measure the humidity and temperature of those colliding air masses.