I don't believe for a second they "need" more money, it's just a classic case of greed. "Much ($$$) wants more.
I think they're knee deep in the almighty dollar, probably already have more than they know what to do with. But humans have a hard time being in the lead without letting it get to their heads. Even the bible warns us from following men, because they're all liars.
This whole Watchtower-phenomenon stems from people not following the biblical advice, not even reading it properly. And all people have a strong urge to belong. To anything. It's just an instinct. And some people have figured out how to take advantage of that.