Simple Things Produce Great Rewards?

As I was reading Get The Most Out Of Production Using Marginal Analysis, I thought this is just common sense. I have a certain amount of money or time and then each of these items are going to use this much of it. I then thought, if it were that easy then why do we have so many companies going bankrupt and closing their doors? So my question is what good is this graph if in reality there are so many other factors that effect it?