I've read that's in part because they were pushing the edge of the state of the art at the time they were designed, sports cars were used as an analogy.

Of course, it remains to be seen if SpaceX's scheme for engine reuse will pan out.

Which is another way in which the Shuttle platform wasn't that much reusable. Falcon 9 and future SpaceX rockets are intended to be just refueled and relaunched, without having to disassemble the engines after every landing.

"Had to be..." is not quite true. How the SSME/RS-25 was designed to operate and could be operated is not the same as how NASA chose to operate it. Particularly after Challenger.