Divinity Engine is just more limited to it's own type of game.
It also comes with all the art assets, animations, sound effects, bgm, scripts, etc . . etc . . . That's the difference between a licensed game engine like UDK and a modding toolset. You just can't compare modding to developing a game from scratch. With mod tools, 95% of the work is done for you before you even begin. That's why most people find it insulting to suggest charging for mods.