Because it's easier to scale hustle then relative humidity. and high relative humidity and high precipitation rate are the associates. I should know because I grew up in both greater New Orleans, which is subtropical, and the Oklahoma panhandle, which is semi-arid 5% relative humidity or higher is normal in New Orleans. While 6% relative humidity is relatively familiar in the Oklahoma and Texas panhandles.
Antarctica is the driest continent; it is almost entirely desert. Very little snow or rain falls on the continent, but because it is so cold, the small amount of precipitation that does fall does not melt.
