Reason 1: We don’t have the time.
If you don’t have the time because of poor planning and unrealistic schedule, there is still hope. But if "don’t have the time" means "we don’t care", there is not much to talk about. But before you go, remember: If you can’t give time to serve your customers, why should they give you their money?
Reason 2: We don’t have the money.
How much is the value of customer satisfaction? What is the productivity loss of not finding the right information at the right time? Are you willing to trade that amount with the cost of usability testing? Besides usability testing is cheap.
Reason 3: We don’t need an outsider.
Outsiders bring an impartial, detached viewpoint that is very difficult to maintain when looking at your own website. Outsiders are also immune to internal political conflicts and they can be honest with you without having to worry about repercussions like getting fired).