A good reason why Equestria isn't a good place to live in for us humans · 7:06am Apr 1st, 2012
Equestria lacks a true natural water cycle.
The sheer fact that the weather there is manufactured, the water vapor has to harvested via tornado and that winter needs to be removed with help from the ponies means that the sun there either doesn't or can't produce enough light and heat to melt/vaporize water.
Oh sure that just means that the ponies have to work together because otherwise they would die horrible deaths by starvation or dehydration!