It's really two different questions isn't it?
1) Why do you pursue money?
I wanted extra freedom. In my 20s I managed to build and piss away not one but two separate fortunes, because all I ever focused on was the short-term -- the thrill of making it and the pleasure of burning it up on doodads. When I had to start over the third time I realised that the thing that I liked the most about having a little money was freedom.
2) Why, specifically, online?
For me it's a no-brainer because it's portable and because of the passive revenue possibilities. It's like real estate that almost never breaks down, can be managed from anywhere in the world, and has a price barrier less than a couple of decent pairs of jeans.
Frank