If I sign a contract with you for 10,000 barrels of apples, and the next day your sole apple vendor decides they don't like you anymore and cut you off, that's

If I sign a contract with you for 10,000 barrels of apples, and the next day your sole apple vendor decides they don't like you anymore and cut you off, that's not my problem. I might be sympathetic to you, but I didn't sign a contract with your vendor, I signed it with you. You owe me the barrels and not needing to worry about the details of how you fulfill that contract is one of the things that makes contracting stuff out useful.