Because Team-Lots-of-Lower-Level-Shmoes can outnumber Big-Bad-Magic-Person by at least 36:1, and single favourite optimised area of effect spell (against whose effect Big-Bad-Magic-Person has also crafted a pro-tip protection device) can turn 36:1 into 0:1 in a single round.
Sorry, trying not to sound harsh, but the entire crew of the Mechane vs Xykon equals Xykon plus ash. D&D just isn't linear.
(OK, while I've edited this, people have responded.... sorry, didn't mean to mess up the responses...)