Because they need to have the highest precision and lowest noise of any amplifier in your system.
It’s no mean feat taking a 0.3mV signal and amplifying 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, hard to even measure. The signal has to be amplified up to a volt or so, that’s over 1000x! This massive 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.
As you can see, this job requires not only a huge gain, but also ultimate precision in terms of parts, circuit design, layout and adjustment. This is why there is such a gulf between cheap op-amp based phono preamplifiers and all discrete class-A tube type phono preamplifiers and moving coil step-up transformers for example. 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. For example, using 1% silver mica and polystyrene film capacitors vs using 5% green caps and other cheap types has a huge bearing on the accuracy of the final result. 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.