Because they are growing in to adults, have hormones raging, and this in itself is tiring. Couple that with the tremendous pressures that teachers and pupils are under to get prescriptive courses done in large class sizes in schools, and they are ready for a break. 30 minutes gives pupils barely enough time to queue and eat their lunch. Many of them rely on school buses to and from school, so lunch time is the only time they have to see a teacher if they don't understand the homework set, or go and see their head of year if they are being bullied, or get a book out from the school library, etc, and as others have said, they should be encouraged to go out and get some fresh air, have a game of footy or whatever during that time.
My child has benefitted greatly from lunchtime clubs and revision sessions. I wish that for other children.