Original Comment by: Evil Timmy
http://10mbit.com
One of the reasons is, undoubtedly, the heavy scripting and amazing set pieces you often get for the finale of a game. If you end up repeating any battle in an average game, it'll be the one against the final boss, it'll involve a fair number of new textures and models, and the map it's set in will be unique and amazing. If the game uses much music, you can be assured there'll be an 'end fight' and 'final cutscene' song. Developers don't want their work to be wasted, and consumers don't want to feel they haven't gotten all the game they paid for.
I'm a fan of adaptive difficulty, but I also think it should be tweakable by the user, basically some sort of bias to the adaptive algorithms. That way, if I feel like getting serious and having a real challenge at the risk of reloading, I can do so; at the same time, if there's a part I find annoying or frustrating, or I simply want to enjoy the same gameplay mechanic a at a lower difficulty, I can do that as well. And I don't have to count on the programmers and a QA team to have created a universal system that knows exactly how difficult to make the same game for every individual who plays it.