I find improving established code of far more interest and reward than greenfield. Coming to understand what a person was thinking while building something is fascinating. My updates are tangible life improvements to real human beings. That interests me, at least.
Greenfield, I have the incredibly stressful job of making all the major architectural mistakes people will complain about for years to come. I'm not interested in that. There's also a much higher chance with greenfield that the product will just fail and no one will enjoy your work.