Why the desire for low magic?

[Mechalich]

I'm not sure the comparison holds up. Magic may indeed be part of the fundamental forces of a fantasy setting but that doesn't mean everyone could or should utilize it. Most people in the real world can't manipulate nuclear reactions without some fairly extensive training, after all.

Vast disparities in character power are generally not viable in TTRPG game structure. All viable PC concepts must maintain a roughly equivalent position on the power curve. That means once one character concept moves significantly past the fundamental limits on the human body and mind's capabilities, others must do so as well. It need not be magic that's responsible, massive technological disparity has the same problem. Conan can't adventure alongside Iron Man either.

The big difference is that the availability of technology to characters in any given setting is usually managed by economic or educational factors. If you 'bamf' Conan into Eclipse Phase he'll need some remedial coursework and a giant pile of dollars to catch up to the average joe, but he absolutely can catch up. Magic, by contrast, usually has gated entry, and the most common form of gatekeeping is exactly the same as most other superpowers: winning the genetic lottery.

However, any setting based around gated powers inherently divides everyone living into the settings into the has-powers/doesn't-have-powers classes (some settings, like exalted and arguably 3.x D&D, have tiers of progressively better types of powers), and unless the powers are very modest - meaning a low magic setting - only the has-powers people are important enough to matter. Which means the setting becomes a supers setting, with all the complications implied by a superhero setting.

And look, fantasy superheroes is perfectly fine. It's very popular, many people love it, it has a thriving literary history that goes back to the literal first recorded piece of fiction we have (Gilgamesh is the literal Ur-super, pun most definitely intended), and it works absolutely great for certain kinds of stories.

However, if you don't want to do supers you have to eliminate the divide. Low magic does this through the simple expedient of making sure the limits on the extant powers mean have-powers group never gets to override the doesn't-have-powers masses (increased technology level helps here by boosting the power level of the doesn't-have-powers group). Trying to do this in high magic means giving everyone powers, but when you do that the result is worlds that bear no resemblance to our own and become massive worldbuilding challenges to put together and tell stories effectively within because you've fundamentally altered the definition of 'person.'