I agree that a phono preamp should be a simple circuit, but that would only apply in an AVR, these days. It's probably on a chip and it really should be easy enough to make these with extreme consistency, but we all know someone, somewhere, has a "better, more life-affirming way". Cartridges don't sound the same, even if you compare a group of the same brand and model- I have done this comparison and it wasn't just frequency response, it was also a matter of channel balance and overall sound level. The person who bought two cartridges and turntables that prompted this evaluation was trying to compare test pressings for the album he was mastering and he wanted to make sure it met his requirements.
Also, one phono preamp may sound different from another because of component variations, so one preamp (admittedly, I'm referring to outboard units but not necessarily high-priced) will often sound better with one cartridge, but that's just a matching issue. If a phono preamp has variable load/capacitance, it's probably going to tolerate a wider range of affordable cartridges, rather than going down the rabbit hole of constantly searching for the best one.