Because it is not a good idea to sleep in TOO late. That disrupts your sleep schedule and will make it harder to get up for the morning on a school day. You will not rest well at night, might find it harder to focus or learn at school.
In other words, it is bad for you to sleep in too late.
Nothing to do? Life is FULL of things to do.
Exercise, read a book, learn a new skill, take up a hobby, help your parents cook and clean. If you find life boring, this is your fault. Life is vibrantly rich is you want it to be.