I can't believe I am the fourth to vote on this.

For me it is the sheer fun of creating challenging flights that represent real world experiences. Plus being able to try something new that I might never experience in my own life.
I have had my private pilot license for 12 years now, and nothing comes close to actually flying a real aircraft into a small airport, in a 7knot cross wind, with power lines at one end of the runway and trees at the other.
I have been using MSFS since the days of FS5, and I have to admit it has come such a long way in a short time. When I am not able to fly the real thing, it is nice to climb into the simulator and go for a spin. It isn't real but for the money, it sure does try.

Plus when I want to fly something other than a Piper or Cessna, the simulator is the only way to go. ;D