That is great that YOU know this stuff already---but what about those that don't?
How would you react if the teacher simply assumed that YOU knew something you didn't--or something that "EVERYONE"
but you already knew--and spent no time on teaching it to you?
What if all the other students were saying:
"dang that everyone--we already know all this stuff and his ignorance is not our problem, why should we pay our good money to have to go back over stuff we already know."
You'd be hacked---you know you would. You would say that you pay your money too and you deserve to learn.
Same with the skills you find so much of a time waster above.