they do it to ease everyone in. would you prefer to go from summer vacation to the daily grind week in week out?
and also the teachers/admins all have to start that monday, so its a regular work week for them, and the students just happen to start showing up on wed once everything is all ready to go.
as for me...
i don't leave for England for another month, and once i'm there i think i have another week before i start classes...and i've been on since early may. Trust me, once you reach college, the idea of having school from august to june is awful. So is the idea of class for 6 hours a day.