Creation of an entire world is long and hard work, and you want to get the most return out of your investment.
Most fantasy authors spend years on building their fantasy setting, unlike SciFi, which is often set in a much closer and more recognizable setting, or even in modern times, or a loosely metaphorical setting.
In the High Fantasy genre, it's considered the norm to build a detailed world, with several continents, dozens of kingdoms, a basic magic system, history, monsters and an epic struggle. If you only write one book, you would a) not be able to transmit even a smidgen of this to your readers, and b) have an awful lot of world unexplored except in the back of your head.
So write a trilogy! Let the epic quest stretch on from sea to shining eel-infested magic-conductive used-to-be-an-ancient-empire-that-sank-beneath-it sea!