Clearly, you've never been to a graduation or even a graduation practice. Both take hours because you have to go through all the students- thousands of them.

We didn't have to do a rehearsal as it is quite straightforward, but my ceremony was definitely around 3 hours! Walking in, speeches, 120 students have to walk to stage, some have extra diplomas that are read out, another speech, a song etc. The rehearsal is likely to take longer because they're still sorting out the what-s and the when-s and the how-s

We practiced for middle school graduation for a week. The person coordinating graduation wanted all of us with our partners to walk in sync.

Think of it as rehearsal for a play. There's your answer. You're welcome.