I think you're not giving Sweeney enough credit. He basically invented modding with ZZT: http://en.wikipedia.org/wiki/ZZT http://en.wikipedia.org/wiki/ZZT Addi

I think you're not giving Sweeney enough credit. He basically invented modding with ZZT: http://en.wikipedia.org/wiki/ZZT http://en.wikipedia.org/wiki/ZZT Additionally, post Quake 3, the team at Epic routinely chose better design tradeoffs for real world games. This includes both performance tradeoffs and spending of engineering effort on production needs (like a solid level editor). Hence the popularity of the Unreal 3 engine.