Well, it HAS to be. There's literally THOUSANDS of configurable settings in the XML files. In fact, I'd say well over TEN THOUSAND.
Do you want to sort through a 10,000+ line long in-game options screen? Do you want TFP spending the next 6 months building that?
The Options screen in game will present a small number of MAJOR options. Everything else (that can be) will be in the XML's for those players wishing to spend the time (and it's literally almost no time at all) to further tweak the game settings.