Because you were just playing on your Kindle. You should be doing your work.

(Daddy is sitting on a stool, working on his computer at the kitchen counter.)

Alex: You haven't sat on that stool, working on your computer for a long time.
Daddy: Yeah?
Alex: You haven't worked a lot at home.
Daddy: You're right.
Alex: Because you were just playing on your Kindle.  You should be doing your work.