Earlier quoted context omitted.
Most good games are made by people who are paid to make them. Or people who hope to pay themselves when their game is finished and sold. It's hard to get a large volume of unpaid volunteers to collectively work together on a creative endeavor. If I'm going to spend my free time working on something creative then it's going to follow my vision and not someone else's. If you want a community driven game nothing is stop…
Whoa - I'm not advocating for people to work unpaid. Don't know where you got that idea. I just don't think anything can really honestly advertise itself as being "mod friendly" or "community friendly" unless it's GPL.
I'm not sure how you could say Source engine isn't mod friendly. Lots of great mods have been made using it.
Professional grade engines are also becoming increasingly available at very affordable prices. With Unity there's really nothing stopping anyone from making any game they want.