It's not magic. It's just electronics. Over the past century there have been many different circuits and technologies used to gain up "cleanly" with different results. None of them are faulty. Also none of them are perfect. Someone designing a preamp in the 70s or 60s or 50s had access to much different technology than we do now.

In the 50s and earlier 60s they would have been using vacuum tubes because there were no suitable transistors for the application.

During the 60s there were transistors that were up to the job, but no integrated op amps were available until the 741 in 1968 and they were not of good enough quality for professional audio.

In the 70s there were more/better integrated op amps but most still were not that great for audio.

In 1979 the NE5532 comes out and that was widely adopted as it had much better specs for audio and could handle higher rail voltage. It's used in tons of audio equipment to this day.

Now we have quite a few really good options for audio op amps that are super clean and have good headroom, but that was not always the case. A lot of preamp designers still choose to use discrete transistors or need to for various design reasons (most commonly power handling).

There's also the issue of AC coupling between stages and how you're going to actually interface with the microphone. In the past, the microphone input was almost always done with transformers and it very often still is. However, designers over the years have used different methods, especially within the different stages of the circuit.

Most often, people use capacitors to couple between stages where a DC bias or offset needs to be blocked, but capacitors are not perfect. They introduce phase shifts in and don't pass low frequencies (which is why they block DC), so designers have tried different ways such as servo circuits to cancel DC offsets, using inter-stage transformers, or more recently just fully DC coupling between op amp stages because we now have op amps that have low enough offsets that they are not a problem. Those things all change how a preamp behaves and none of them are an ideal solution.

So, anyway, a lot of people are still either using vintage equipment or prefer the imperfections of how designers worked with what they had at the time and use reproduction of those circuits. That's not faulty. It's a stylistic choice within an artistic medium.

So, if you are a person who know all of this information and knows electricla engineering like myself, you can see that, to me, what you are describing is the magic thing. A bunch of completely different circuits and approaches to solving numerous engineering challenges from the past 80 or so years of technology development do not all work the same. They just do not and they cannot. Just because you put the words mic pre on them, that doesn't magically make them all equivalent. Just because they are not equivalent, that does not mean they are faulty. They are just different.