For some of the really AU stuff, some people do it because they want to use certain character archetypes and tropes within their story. But for a lot of it, if you're using a preexisting character and familiar setting or elements from the preexisting canon, then you don't need to spend as much time world building. If you just want to tell stories to a wide audience, then writing an AU fic in a popular fandom will get you more exposure. People are already familiar with the characters and are usually looking for dynamics involving those archetypes.
Some people just want to explore the what-ifs, ranging from timeline diversion to crossovers.
And then you also have those people who just throw the character archetypes out the window, and in the end all they're using is a name and physical description for their fan-casted story.