Tbh, it's pretty arbitrary since the alternative would be to gave 1 day be several minutes, which would mean thay other things would also have to take much longer. Animals take years to reach adult, so if days were longer than that means that animals could take hours to reach adulthood by default (and this is already possible through settings).
If we want to go deeper, the day/night cycle isn't synced up with the days either so it could be morning for about a whole month, or guests staying in a zoo for several weeks, or the zoo never having closing hours. It's pretty much just the suspension of disbelief
