they exist. i work for one. the engine was built as we developed worlds on contracts for clients. once we got it to a certain point, we started licensing it out.

we term it a platform, as its something to build with an upon. all of the projects we've seen require extension and addition to the basic model we've defined to customize it for the project at hand. but we've baked in things that make doing so trivially easy.

http://www.electrotank.com/corporate.electro