As an audio engineer (self-taught, but the same skill-set), I can say from experience that the live environment is much different from the studio. Audience interaction is a big part of that, but one of the greatest technical differences is "stage-bleed".
Lots of instruments have their own acoustic volume that escapes directly into the audience and can't be turned down: uncaged drums with a "cymbal basher" playing them, screaming guitar amps that the musicians refuse to turn down or off, etc. There are ways to get the same sound into the PA with a silent stage or nearly so, but they're not always accepted.
Thus, the Front-of-House Audio Engineer has an absolute minimum volume coming directly from the stage, that (s)he has to mix around or in some cases drown out. (like the "pad-slap" from an electric drum kit) Fading out the PA allows the "stage bleed" to perceptibly take over, and that usually doesn't include enough parts to sound good when the entire band is meant to be amplified.
As a side-note, this stage volume is also a problem for getting good vocals and other things that must use a mic. Mics don't know what you want; they always pick up everything that happens to be in their designed geometric pickup patterns. So the choice of mic is not entirely about the tonal quality, but also about the pickup pattern.
There are a few tricks to minimize interference, like putting monitor wedges (speakers on the floor, aimed at the performers and controlled independently from the main PA and usually each other) in the null-point of the most critical mics' pickup patterns; but they're not perfect, like the same monitor reflecting off of something and coming in from a non-null direction for the same mic.
And then we have performers that insist on waving their mics around and/or holding them in a way that covers some of their ports and thus destroys the designed pickup pattern...
So we engineers have another motivation to keep the stage quiet: it makes our jobs SO much easier! And it allows us to not criticize the talent as much for eating up all of our barely-existent technical wiggle-room with their artistic shenanigans.
In-ear monitors (IEM's) are WONDERFUL! Essentially a set of earbuds, usually custom-fit, that are controlled in a similar way that the wedges used to be. However, I have also heard, quite often, a click track (metronome) getting into a vocal mic from a too-loud IEM, so they're not perfect either.
If the stage really is that loud and can't be helped, then the screaming vocals that metal is known for, become necessary from a technical standpoint as well as artistic.