Do a simple search for an RPG on Sourceforge, and I think you will see that over a quarter of them are "engines" (and don't get me started on the number of unfinished projects). I think the reason for this is simple - its much easier to write an engine as you don't have to make it a product that end-users (the most discerning of all people that use software) will have to use. You only have to appeal to other developers, and they are normally experienced enough to get a difficult piece of software running, as they understand phrases like "compile" and "libraries", and it means you can write something that doesn't have to be finished.
Don't get me wrong - engines do help. Almost any piece of software written these days requires some library or engine to run. But how many people are just re-inventing the wheel? And not even the full 360 degrees of it?