Because as much as spending 4 hours doing unpaid work may seem to suck. (Which let's face it, I do at least 8 hours of unpaid development a week on side projects.) It's a better and more accurate test of your abilities than doing what can only be compared to that of a trivia based game show.
Option A) Spend anywhere from 2 to 12 hours interviewing for a position with little to no break. While being asked to solve problems, that may very well actually be unsolvable, under the pressure of rotating team of people. And for bonus if you suffer from any sort of test taking anxiety in school you'll get to feel the blood drain from your prefrontal cortex, as your fight or flight instincts kick just enough to make sure that doing high cognitive work like programming is next to impossible.
Option B) Spend a few hours solving a hopefully interesting programming challenge at home where you're probably not wearing pants.