Why do better MC cartridges cost a fortune?

They are the emperor's new clothes.

Even so, any decent cd player will be superior. Turntables are just money spinners for the trade, and if you are like me and you like mechanical engineering, they are also nice toys.

The problem with old cartridges is that their suspensions may have dried out.

There is the pretense that there is something (audio quality) which there is not. The analogy is with the listeners/onlookers who sheepishly follow the flock.
Of course there are recordings that are not on cd and only on lp. I still have my turntable for those, but I find I rarely if ever use it. The reality is that streaming services now offer such a wide selection that no private music collection could ever compete. I have a busy job, but if I had time I would digitize all my lp's and just get over with it.

And yet, none of them are as neutral and reproduce the original as cleanly, with as little distortion, and with as much channel separation as a cd/dvd/bd disc player of a few hundred dollars at most. As I said, such cartridges are the emperor’s new clothes.