The main reason why it is profitable to establish a firm would seem to be that there is a cost of using the price mechanism. The most obvious cost of 'organizing' production through the price mechanism is that of discovering what the relevant prices are.
There is no doubt that the recognition by economists of the importance of the role of the firm in the functioning of the economy will prompt them to investigate its activities more closely.
Roughly speaking, when you are dealing with business firms operating in a competitive system, you can assume that they're going to act rationally. Why? Because someone in a firm who buys things at $10 and sells them for $8.00 isn't going to last very long in that firm.