The fact that people don't do what they should doesn't mean they don't know. People know they shouldn't eat tons of food and gain unhealthy amounts of weight. People know they shouldn't smoke. People know they shouldn't drink to excess regularly. Still people do these things pretty frequently. It's not a question of ignorance but a question of will, or priorities, or maybe ability.

Go ask any general practice doctor whether people do what they "should" and whether the reason when they don't is actually ignorance. People have lots of trouble prioritizing long term benefits over immediate pleasures.