Because sprinting is a whole body exercise and the type of training often means that some hypertrophy will be required throughout the entire body (you need a strong upper body to clean, squat, deadlift, bench etc). Also think about it - how many people do you know who have big legs do not proportional upper bodies (unless they happen to participate in something like cycling).
lol i have big legs and a small upper body! but then again im young and havent been training for long. but sprinters need core strength etc, and need to be very strong and powerful.