If people had the right attitude towards food to start with, these products wouldn't be available.
Some people need them to help get the right attitude.
Joe down the road wants to lose weight but has no idea about nutrition etc, can't afford to see a specialist so trys the meal replacement diet, sees it's working, replaces a shake with the same calorie meal and after time, learns about nutrition and develops the right relationship and attitude. Without those shakes, Joe may have stayed overweight because the encouragement gained from seeing results will not have been achieved.
People with anorexia use them. It's not food so they feel better consuming the calories and eventually replace them with food. If you try giving an anorexic person food, they will refuse to eat it. This helps.
Then there are people like myself who find the act of eating disgusting and cannot do it. Most days I can manage to eat a meal but the thought of eating three spreads fear beyond belief through me. I need nutrition, I need meal replacement shakes. (I am working through this in case anyone was wondering. I've gone from eating nothing to having five/six meals a week which is huge.)
Saying people need the right attitude towards food is like saying people shouldn't be addicted to drugs, gambling, etc. They know it's wrong, that they shouldn't do it and it's damaging to them and their loved ones but they can't help it. There is something inside them that compels them to do it. It's the same with food. Joe knows he shouldn't eat that whole pizza for lunch, the anorexic girl knows she shouldn't through away her dinner and I know that eating food is not disgusting but that doesn't stop the overwhelming fear I feel every time I have to eat. It's not always as simple as eating the right foods at the right time, it can go much deeper.