Why Do People Not Know When To Heal or Back Off?

Its getting straight retarded out there. I cant even name a specific champ because I see it everywhere different champs but mainly on the carries.

People out there 1 or 2 hits away from death and they just stay out. I mean iI can see if you have lifesteal and you hit a camp or a wave and fill up but you got people with no life steal acting like they are playing aram and can only base when they are dead.

What led to this post was a Zed in my last game...repeatedly refused to go heal.

And why do people act like the other team will stay dead forever? You win a fight and take a tower and even tho you are low health you stay around like the other team will not spawn with full health? And then you die and your like "wtf!? so and so is fed" No their not fed your ass just stayed out with no health and got jumped on by 3 or 4 people.

Or when you should be running you keep juking back TO the enemy like they are scared of you and your 100 health.