Because in order to faithfully amplify the minuscule signals generated by a phono cartridge, they must have the highest precision and lowest noise of any amplifier in your system.
It’s no mean feat to take a 0.3mV signal and amplify it to the level needed by a regular line-level preamplifier. One millivolt or 1mV is one one-thousandth of a volt. Moving coil cartridges typically have outputs of less than 1mV, a tiny signal.
This has to be amplified up to a volt or so, that’s a gain of over 1000x. This amplification has to be made whilst adding as little noise and distortion as possible. The phono preamp also has to EQ the signal to RIAA specs, reversing the EQ applied to the signal embedded in the record grooves.
Audio Microscopes…
I studied botany and zoology at uni and am well-versed in microscopy. The microscope is a perfect analogy in optical terms, to a phono preamplifier. Ever used a bad microscope? The image is distorted, lacking resolution and opaque. It’s exactly the same, only in sonic terms, when you use a bad phono preamp.
The job these instruments do requires not only huge gain (magnification in optical terms) but ultimate precision in terms of parts, circuit design, layout and adjustment (lens quality and figure, low optical distortion). This is why there is such a gulf between cheap op-amp-based phono preamplifiers and discrete class-A tube-type phono preamplifiers. Its also why a kid’s microscope is $50, vs tens of thousands for a really good one.
Step up (no pun intended) to a good moving coil step-up transformer for example and the price hike is considerable, likewise, to a good class-A phono preamp, or a really good microscope. Parts like JFETs, big film capacitors, precision resistors and premium wiring all add to the cost of these instrument-grade amplifiers.
I should mention that some of the op-amp-based solutions aren’t even that cheap, but moving to a precision, discrete design always improves things. Likewise, ramping up parts quality and grading has a profound effect on performance. Using 1% silver mica and polystyrene film capacitors vs using 5% green caps and other cheap types greatly affects the final result’s accuracy. When you are dealing with such small signals, you need accuracy.
The best phono preamplifiers use MKP (polypropylene film), silver mica, polystyrene film, discrete transistor networks and tube gain stages. The very best use transformers for the critical job of boosting moving coil signal levels.