The economics of the game engine business are terrible. The economics of the game business in general are terrible already and when you're in the engine business you've got a big cost overhead with a revenue stream made up mostly of 0, a few small trickles, and then some big wins which turn out to be capped licence buy-outs, because your customers are in a business with poor economics. It's worse than selling to Apple. AFAICT (I have no special inside knowledge) Unity isn't making any real money (I believe even a company the size of Microsoft pays only $2M/year, though they may also pay per-title fees).

As far as I can see the only way it works is the way Unreal does it: they develop their engine for their own needs and coincidentally let others do it (cf Lumberyard). It's conceivable that a "casual consortium" a la GCC, Linux et al could build one but I don't see any signs of it.

  a very steep power law, high marketing (and even licencing) costs, heavy competition...  It's a $100B market but most of the profit is around the core product, not in it.