For me it boils down to cohesiveness. If you take the best/classic albums,(and not just in terms of hip hop, but I'll give hip hop examples) that are quote/unquote five mic albums, the album is a cohesive unit. I'll use ATCQ's The Low End Theory as an example. How many producers produced this album?? One. The group itself, with one track done by Skeff Anslem. Who was featured on the album besides the group? Brand Nubian & Diamond D on one track, and LONS on another. On a lesser extent, we can also throw in Vinia Mojica on the la la la's, and Ron Carter on the bass (both on the same track). Yet the album is all about the group. No 10-12 different producers and 20+ feature shit like today's hip hop albums.
Infact, if you take a look at The Source's "Five Mic Albums" list you will see that most of the list followed this model.
Albums That Originally Received Five Mics:
People's Instinctive Travels and the Paths of Rhythm -- A Tribe Called Quest
Let the Rhythm Hit 'Em -- Eric B. & Rakim
AmeriKKKa's Most Wanted -- Ice Cube
One for All -- Brand Nubian
De La Soul Is Dead -- De La Soul
The Low End Theory -- A Tribe Called Quest
Illmatic -- Nas
Life After Death -- The Notorious B.I.G.
Aquemini -- Outkast
The Blueprint -- Jay-Z
Stillmatic -- Nas
The Fix -- Scarface
The Naked Truth -- Lil' Kim
Trill OG -- Bun B
Even though Illmatic (the exception to the rule) had a few different producers, it was still cohesive because they were NY'ers who understood Nas' beat needs, plus he worked with the best.
Also hip hop is the only music that's sold cheap. The rap game is just like the crack game. Does Third Eye Blind, Oasis, The Rolling Stones, or even Sade and Stevie Wonder have to make 2 or 3 mixtapes before they're album drops? No, only rappers are forced to give away music like 2 for 5's on a street corner. So in turn it cheapens the value of the music, which is why we hear what we hear today. A thousand & one niggas that call themselves producers (with 3 chord melodies of course, made with ProTools, or FL Studio), and a million & two niggas who think they can rhyme, but lack any sort of talent, and moreso, direction to make anything cohesive.