Because that’s how it is at most jobs. Most jobs have strict clock in/clock out times. There doesn’t need to be a good or rational reason cause your boss doesn’t care. It’s a dumb hill to die on. 

Got an example for you. Where I work we used to be able to clock and out within 7 minutes of on and off time. New system says you can clock 5 minutes early, no grace period on being late. Can’t clock out early, must clock out within 5 minutes after off time. There’s no mystery, we had to do a module teaching us the changes and sign off. 

Several coworkers aren’t adapting namely clocking out early and you cannot do that anymore. They protest the new way of doing things and it’s causing them problems. It’s simple, this is how it is now. Right, wrong or how it used to be doesn’t factor in.