Of course, sprinting is better, but a person cannot sprint for 3 minutes. The most difficult sprint discipline was the 400m (43 seconds world record), because it is also the longest. In recent years, in athletics, the 800 m discipline has become a sprint event (1:41 world record). If athletes can't sprint for 3 minutes, neither can boxers, nor recreational athletes.
THey don have to sprint for 3 minutes as theyŕe not throwing punches non stop for 3 minutes either. Sprints help with speed and explosiveness and are also anaerobic, so is boxing. Between sprints, jumping rope, sparring etc a fighter can get all he needs.
When you are running rounds, you run like you fight. So there are times when you jog, times when you sprint, if that makes sense.