We do because when we're growing up, no matter of ego will give us the abilities that we think we have as kids (i.e. that looks easy I can do that easily, or, I can run a house with my better rules on my own). We have to lean on our parents for them to teach us how to do things in life.
The psychological problem with being "OMG TOO FAT" is only the tip of the iceburg which is why your message is not being received at all. The overall perceptional of the user's own body being fat leads to the following thought in their minds:
To control one's eating/body means to control one's life.
Once that main idea sinks into their persona, then the real psychological mess manifests itself into them. Their delusion of being fat leads them to the fear of them being not in control of themselves and will go to extreme lengths to get it back into their distorted definition of "control". This typically involves hardcore fasting and hardcore exercising, and hardcore purging (voluntary vomiting). This sudden demand on the body without any fuel (except for muscle and bone...and yes...muscle and bone fuel exercise come before fat...he/she needs to know this important fact causes some high stress on the person.
As she loses control of that to that stress, she begins eating/drinking for comfort. Eventually he/she realizes "OMG I'M EATING" and begins the vicious cycle of trying to gain "control" all over agian. The important objective is to show her that the disorder is controlling her, not her controlling it.