Tvad, that is an excellent question. Adam18 has basically answered it for me, as well, thanks. I have heard that if you want to try a different cartridge on a Rega, you have to buy their spacers to do so, as Rega's cartridges are not quite the standard size. Another reason for the after market stuff is that in this hobby, as I am sure you are aware, there are a great many people much more interested in the equipment than the music, and these folks usually can't leave well enough alone, they have to keep messing with stuff.
I should emphasize that this is NOT a criticism of the great many DIYers who use musical reasons as the basis for their experiments. I would love to build my own SET amp, for instance, or my own horn speakers like my uncle did, if I had any engineering skills whatsoever. Maybe after I retire, but that's a long way off yet. In the meantime, I figure that the manufacturer knows alot more about it than I do, so I will not mess with my Rega setup.
I should emphasize that this is NOT a criticism of the great many DIYers who use musical reasons as the basis for their experiments. I would love to build my own SET amp, for instance, or my own horn speakers like my uncle did, if I had any engineering skills whatsoever. Maybe after I retire, but that's a long way off yet. In the meantime, I figure that the manufacturer knows alot more about it than I do, so I will not mess with my Rega setup.