We train our engineers to build airplanes, then we ask them to build rockets, is it any wonder that our rockets look like airplanes stood on end.
So, take a brick, one pound in weight, and stick it somewhere in the airframe of a Boeing 737. How much is it going to cost you? Just doing some back of the envelope figuring, with current fuel costs, and over the long life span of a 737, about $1890.48. Pretty expensive brick. It would pay you to have somebody go look for that brick. If you have a fleet of Boeing 737s you should hire somebody to make sure that people don't leave bricks on your aircraft. You should certainly demand that Boeing remove any useless brick shaped objects from any new 737 you might want to buy.
The problem is that on an airplane you're going to haul that brick around for many years and many many miles. On a Minimum Cost Design rocket you're only going to look at that brick for a few seconds on the booster, and a few minutes on an upper stage. That makes a big difference. On a booster that runs for 50 seconds, the brick will cost you ........ 10 cents. Not even worth worrying about. A ratio of 18904.8 to 1. Of course if you noticed someone loading a truck load of bricks onto a MCD booster you might be slightly concerned.
In structure, the Apollo Saturn V is very similar to the Boeing 747. They were both designed at about the same time by the same people, aeronautical engineers.