Even in an ideal world where there's no fundamental difference in the cost of producing 1000 units or 5000, companies aren't going to be able to sell 5000 at the same price as they could 1000 when doing so exceeds their current capacity. Expanding that capacity means more staff and equipment, which costs money that has to be paid for somehow. It also takes time, which means that it cannot act as an immediate solution to the cost problem and also that there's substantial risk involved - by the time the extra capacity is there, the demand may not be. In order to justify taking on this risk there needs to be the expectation of substantially higher profits.

Also, commercial airlines in particular are an industry with a history of over-optimistic expansion followed by cut-throat competition on pricing and then bankrupcy, and I'm pretty sure everyone in the industry knows this by now which is why there's not a huge rush to expand massively.