That too. There's not alot of variation in desert climates, it's pretty much clear and sunny all the time. Which is what it's like throughout most of the Wasteland, aside from that green fallout in the sky. While it does cloud over in certain areas, clouds don't necessarily mean rain.
True, however deserts are notorious for having flood-level storms from time to time. And they are pretty damn intense. If i had to imagine that in Fallout 3, I am imagining my character being dragged into a Radio Active body of water and drowning lmao.
Odds are, alot of people aren't running "graphically intense" PCs (I realize this is for PS3 and Xbox 360 as well, but if you left out rain for only the PCs, PC gamers would revolt and quite possibly burn Bethesda HQ to the ground xD). they probably decided to leave it out, and figured it wouldn't take long for the modding community to add it in, and it didn't. There are a few different mods which add this feature.