I saw this since I was playing Sims 4 for some time and I was in a german sims group, asking why they do it and one girl just answered, quote: 'Well, we put afford in our creations in contrast to Sims 3 players and we got the right to get our work paid, its our time that we put in the creations, do you really think we just do this shit for fun?!'
I was really shocked. Yes, I think that many modders do this stuff because it's a... well... hobby and even without them paying for their stuff! I could rage about this all day.