Why is the workout different every day?

The workouts (WODs) are different each day because that is part of the “magic” of CrossFit. Constantly varying what you do keeps your body guessing, unable to adapt and plateau like it would with a structured, regimented workout program that works body parts on scheduled days and does not vary much week to week. When you consistently follow the same running route, work each body part on the same day each week, do the same exercises on the same day each week, etc. then your body will get used to what you do. When that happens, progress slows and you fail to see the progress that you once saw.