Except it kind of was. Car companies don’t manufacture the most complex components in house either.
Those things were custom, but NASA saved a great deal of money going with an existing supplier of giant rocket engines who had already been working in scaling up their designs.
Most car companies do indeed produce their own engines. It's why many of them are named "XYZ Motor Company".
Of course, they don't produce their own microchips for all the electronic stuff in cars these days, and I suppose you could say those are more complex, but back in the 60s when the Saturn V was made, the engines were probably the most complex things in cars.
Over half all heavy duty trucks in the 1960’s used Cummins engines. Their engines made their way into Doge RAM trucks etc. Aircraft manufacturers also tend to outsource engines as do shipyards building boats.
Most major car companies did and do manufacture engines in house, but it’s common to find some outsourced engines and various joint ventures used by automotive manufacturers.
Well of course a bunch of ultra-tiny car manufacturers are going to use outsourced engines. The major automakers never have, outside of a few odd projects (like the Ford Taurus SHO with the Yamaha engine) and joint ventures.
Heavy-duty trucks and aircraft are very different situations.
