It's not about 1 child being late. It's about group expectations and safety and attendance.
Imagine if the kids could turn up whenever they want without recourse... You're taking morning registration ( a legal requirement) just before you start your maths lesson, and you have no idea whether the child is going to walk in 15 minutes late or if they're missing.
You have to report missing children to the office staff. So you report them absent without reason, and then staff have to go and fine 13/30 kids who just decided not to bother showing up.
And then, as you've started the lesson for the 17 present children, the 13 others start appearing in dribs and drabs over the next 20 minutes, even if they "sneak in quietly", each one is disturbing the lesson, having to catch up on what's going on. All the children will be distracted every time another pupil turns up.
Bells rings and the kids leave, and are replaced by 15/30 for the next class. You take the register, report 15 kids missing to the office, the staff try and find them and hurry them to lessons... then you start the lesson, and 5 of them burst in 13 minutes late, giggling and causing a kerfuffle whilst sitting down, you get them settled, another 2 turn up, then 5 minutes later 4 turn up demanding to know what's happening in the lesson, then 1 turns up and starts calling across the room to their friends etc. oh and they still haven't found one kid. Just because they felt like not going and there's no obvious/immediate consequence to them.
All in all, It's a sensible thing to insist children are present on the room by the time the lesson starts.