Insurance is fundamentally lopsided in the risk of a bad decision by the insurer. They have little incentive to keep you as a customer, since the potential profit is just not that big. And thus they have little incentive to do a thorough investigation of potential problems. It's just not worth it. They'd rather dump you, or risk forcing you to replace a perfectly good roof, than put effort into determining whether their report is accurate.

It's a funny business. Every insurer wants better risk assessment, since it's a competitive advantage. At the same time, the better the risk assessment gets, the less point there is to having insurance in the first place. At the limit, insurers that could accurately predict the future would charge you premiums equal to your actual future costs and you might as well just put the premiums in a savings account instead.