To escape reality.
Real answer: They have this need for creating a story satisfying themselves with their creation, these people’s heads are full of imagination (or lust probably). Majority of the writers about this story are male, ranging from adolscent to young adult (take it with a grain of salt). These people are still young, or at least their mind is, so they are still creating this fantasy of people living with 900 women.
The bad thing is that if the writer is older than 20-ish, they use it to escape reality. Having no gf and all of that. OR satisfy the readers. If he chose the latter, then he's making big bucks.
Conclusion: they do it to satisfy themselves and readers'