Why don't people flush public toilets

A lot of times at school, I see toilets full of either pee, poop, or period blood, (Or a mixture of all three).

It makes me wonder...why don't these people flush?
hahah this is a serious question. Do they forget to flush? Are they doing this on purpose? Are they trying to conserve water?

IDK but people need to start. And if it's too gross to touch, use your foot to flush. Just flush the damn toilet!