If you sell a new control system but there is poor airflow to one or more rooms, the customers still won't be comfortable. You will not solve their problems, and you will not gain their trust. New equipment will only be a Band-Aid on a much deeper, more insidious problem.
Part of the problem is ego. If you find out that your own work isn't as good as you thought it was, that can be hard to take. Finding another company's errors is a little less bruising.