Having a roof inspected when you purchase a home what is the inspector supposed to look for and how?

My wife and I purchased a home two years ago. We hired an inspector to check everything in and outside of the home. Recently I found shingles laying in my front yard. I called the insurance adjuster out to make a claim and he said it was a maintenance issue. He said the entire roof was rotted underneath, had three holes, and the flashing by the chimney never had tar paper or shingles on them.he also said without a doubt the roof was like this preexisting when my wife and I bought it. I called three different roofing companies to look at my roof and they all agreed it has been like that for awhile. I called the inspector to see why he stated on his report the shingles are starting to curl but the roof is in sound condition. The inspector said when they inspect a home they don't even need to get up on the roof to look, they can give there report from standing on the ground looking up at it. Is there any truth to this? Should the inspector be reliable for not letting me or my wife know that the roof needed to be repaired?