Insurance companies are not required to negotiate with contractors... it is just that simple. Building contractors do not have experience in interpreting insurance policies and how they apply to a loss. They are not knowledgeable in calculating depreciation, coinsurance, "actual cash value" and "replacement cost value" provisions of the policy, and other elements of your loss. Further, a contractor is competitive with his bid, attempting to provide his lowest bid and often minimal new materials in order to be hired for the job, thereby probably reducing your maximum and legitimate financial recovery, perhaps significantly.
Hidden heat, smoke damage, and water damage could cause you problems, long after you finalize your loss. These items and many more, are usually overlooked because many contractors who are unfamiliar with the methods of preparing a detailed estimate in a precise computerized format which the insurance company recognizes and most often requires. In addition, a contractor can't handle your contents inventories, or prepare your additional living expense or business interruption claims.
Professional Public Insurance Adjusters engage the services of skilled, knowledgeable, experienced engineers, builders, inventory evaluators, sometimes even architects and/or loss experts to prepare highly detailed estimates of all aspects of your losses. The building estimate needs to be in a computerized form and content familiar to insurance company experts, and is done in conjunction with examinations by both the company's representatives and your Public Insurance Adjuster's experts. Every item of reconstruction is accounted for and detailed. The estimate specifically details all aspects of damage repairs necessary to pay the full amount necessary to allow you to restore the building to its original condition or evaluate it as to its current value. And then you need to consider your replacement options... rebuild your home or business building where the loss occurred or elsewhere, replace it with another structure in another location or State, etc.