You know, I have experienced whole weeks in RL in which there was no rain. That more than encompasses the supposed time frame of AO and AE. Maybe the reason was, rather than a devastating meteorological phenomenon, it was simply a short dry spell.
Then again, many levels were indoors. Who's to say it didn't rain then?
The functional reason was that there was no need to introduce such weather to the games. It may have even been a technical impossibility.
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