Imagine you are SpaceX. You are going to be launching a Starship. Given how booked your launch manifest is, would you rather launch a cubesat or a full-sized payload? Which would earn you more money?
Starship still needs to fire rockets in order to land. I would say it's a crazier idea than the ones you have mentioned. The previous solutions you have listed were used precisely because they were less crazy ideas.
I mean, the whole statement is not correct. Sure JPL manufactured some things but it also acts just like NASA, managing the Mars projects. Most of the spacecraft I would say were manufactured in partnership with other industry aerospace companies.