Maybe they have yet to discover it. I have a LP style guitar with a PG set. I don't think the neck PG is muddy at all. I don't use real heavy amounts of gain though. What it does have is a thick, meaty tone, which I think a player looking for heavier tones would want, and a bright, slicing attack. At low gain levels, it has a nice bluesy ping, but with a meatier, thicker sound than a Strat pickup. The PG is a little fatter sounding than a 59.
The Full Shred neck is also great. I have those in a couple of guitars with LP scale length, though they have Floyds where LP has a TOM bridge. The Full Shred neck sounds flutey, also with a slicing attack. It reminds me a lot of neck pickup tones in some Iron Maiden solos.
Another choice for a fat sounding neck pickup that works great for heavy music is the Dimarzio Air Norton. It's really a unique animal. Has a bit of ping on the top, a very thick midrange, but a bottom so tight that you can play power chords with it in the neck position and they don't sound overly bassy. I have one in a Hamer Sunburst (LP style construction with double cutaway). It makes a very warm, fat but tight neck pickup.