With a cartridge, you only have a ground connection on one end. Equal currents induced in the same direction on each wire (common mode) "induce" the same voltage and cancel each other out (it is a loop), at least at practical audio frequencies. Differential noise, will of course look just like a signal. The cartridge is already "isolated".
That is true, as long as currents flow in the loop only. If one of the wires go to input while the other goes to GND (unbalanced input), currents in both wires are not even anymore. Perhaps, that's why floating (for audio frequencies) balanced output, like transformer is not enough and has to be connected to balanced input (not single ended). Microphones also benefit from balanced input.
As for Instrumentation amps, yes they have laser trimmed resistors, but front (two amp) differential section has always gain of 1 for common mode signal independently of resistor tolerance. If you set gain of 100 for this section you get automatically 40dB CMRR independent of resistors tolerance. It happens because each of two amps is referenced to input of another (instead of the GND).
https://www.researchgate.net/figure/Fig-Two-Op-amp-Instrumentation-Amplifier_fig1_299514235That is the only way I can see, for truly balanced amps, to function without converting common mode noise to normal mode signal. Otherwise matching resistors and keeping them matched to some sensible number (like 60dB=0.1%) is not practical, while adding RIAA frequency correction and matching capacitors to 0.1% is next to impossible.