Why do people invest so much time, and sometimes money, into Second Life?

From what I have learned, people, in real life, are in some cases quitting their real life jobs and "working" based on what they can do in Second Life (SL). Some people but islands for $1,000+ and then pay around $200 a month for "maintenance" fees to Linden Labs, the people who developed and run SL, and then rent off plot of the island for people to use, anywhere from $10-$80 a plot. Others develop different objects in the game for users to buy, like clothes (skins), furniture, entertainment items, etc. In the more recent years, SL has been growing so rapidly that the users have had more of a demand for certain items that the people who are developing them are able to quit their jobs, and in some cases, make more money by producing services virtually than when they were working in real life.