How? It was accessible through a door. Nobody - not the seller, agent, himself or any other prospective buyers, or the building inspector he presumably engaged to check the place over before signing contracts - thought to look behind the door? How can you buy a house without checking out the foundations/basement yourself or by a pro?
Whatever the inspector finds, whether it be $50k or $100k or $200k worth of repairs that you request as a price deduction, there’ll be someone else who won’t care about the inspection issues. It’s in your best interest to make the sellers life easier and execute the fastest sale.