If you want fruit, eat fruit. If you want a drink, drink some water. Why combine the two?
Fruit juice seems like basically a way to drink soda but with the delusion that it's good for you.
1) People know what's good for them
2) People want what's good for them
The existence of an economically viable product such as juice indicates that whatever the industry is doing, people want in its presented form.
Whether it's good for them or not is a completely different question.
Sure, the stuff tastes OK, but not as good as the fruit it comes from.