This is a good point.
And let's also consider how much of what is "well-documented" focuses only on the most sensational aspects and nothing else (especially where media, humor, popular culture, etc. are concerned), such that things are reduced to utterly trite fast-fact nuggets to be regurgitated ad nauseam by boring assholes roping people into boring conversations and rendering subjects faceless parodies which fail to hold practically anyone's interest anymore in the process. Things are "well-documented" to the point of irrelevance. People are stuffed sick with "well-documented" sensationalism and mythologizing. How well-documented can something be when people only want to focus on entertaining pop-corn facts? Most people don't actually care about the mundane things in between.
As diverse and deep a resource as MA is, it's not like 90% of the shit on there isn't submitted by bands themselves, or some guy's nephew. Most of it it still goes unnoticed. Most of it ought to. Also, as an umbrella term, "metal" covers a massive array of styles with a much larger collective fanbase. Plus, it's only natural that a bunch of metal dorks would waste the time to catalog so much of it. Now, where's the "Punk Archives"? That's been discussed at length, years ago, on MA itself. Metal is a genre for dorks, ultimately (not a criticism, but a fact). You just don't get the same degree of obsessive dork-passion with other musical genres, otherwise, they would've gotten around to setting one up already.