I think this is an individual type thing where the person has to give you their reason why. Most people do it for the money or because they love their idea.
For me, it's because I:
-see a need that I can fulfill
-want to offer something to the world
-hope to make some decent money from it, but not my primary reason
-want to see if people will use my invention
-desire to make someone's life easier with my invention
-I'm just a creative minded person and I can't even rest until I create something to help others