All the usual reasons to do any job. Money (but not too much
), personal and social satisfaction, the "family" of the shop, ego (knowing more about something that others who are smart don't). Then there's the less common reasons like not being able to fit into the typical work place/corporate world, being financially independent and needing involvement with the world and you love riding. Andy (of course there's many reasons)
To continue I suspect the not asked question is how does one continue to do this stuff with real world pressures to be self sufficient/successful.