As a consequence of people entering the program at different times, required classes and electives are a blood sport to schedule. Some courses are only offered in fall or spring with a prerequisite class to be completed to schedule, so you could have to literally wait and sit with a very light schedule for a semester because you were a half semester off the 4 year trajectory, or the class was full and you have to wait until it's offered again. This ultimately wastes a lot of your time at college. I bet if scheduling was completely painless and not constrained around classes with a waitlist double the size of the class, and you could take your required classes any semester you'd like, people could get the CSE degree in 3 years and save quite a lot on tuition.