Because it doesn't mean what people THINK it means. It's not a chance %, odds, of getting rain that day. Rather it is a historic %. It takes into account patterns past that are similar to the one you will be experiencing that day and gives you THOSE odds for seeing rain on any given day.
So if they say 40% chance of rain, it means that 40% of the time in the past when situations were similar, it rained.
So if they say 40% chance of rain, it means that 40% of the time in the past when situations were similar, it rained.