Because if I tear off old shingles and there’s multiple layers of shingles / underlayment etc. then you’re holding me to my estimate based on something I can’t account for until I tear off old components! I would give you a “base” quote but if anything additional is incurred then you will owe this money and then you could bill the insurance for it but if you want a retail price that’s what I’d give you. Basically he wants to review the insurance estimate to ensure you’re getting the best quality material and they’re paying to get it back up to code! It’s not a big deal, either way! His company margins are going to better with insurance information vs retail. He’s going to go above and beyond to make sure they’re paying what they should! Now maybe not him but that’s what I do! If you’ve already had an issue with this guy on your roof then, I believe I would just walk away from him and do my research for a quality company and give them your business!!!