Because to be honest, I think they take away from the beauty of the car, and if I got tired of looking at it and decided to peel it off...well.... Putting it on a magnet gives me the option to go back and forth without harm to the paint. Those magnets are so thin that anyone looking at it don't know any different. Plus it gives the next owner the option, as I find most people I'm dealing with on these restorations really don't want things stuck to their cars that the assembly line didn't install, lol.
Back then, and even now, I always considered that stuff as free advertisement for the dealer anyway and was never interested in having that stuff on any of my cars. Displaying it with the car along with all other documentation however all in one place I think is a nice way of doing it, so you can take it all in at once.
On the other hand, yeah I see the attraction and completely understand why some would do it. Me personally, seeing it on an unrestored car, now that's cool. Seeing them reapplied on a high end restoration, not so much.