Well, stop and think about this...
On One Page, you could fit every rule about grappling a creature. A simple chart of modifiers, notes on specific feats that aid this, and a summed-up and Errata'd/Sage Adviced commentary on it. (Ok, maybe a two-page spread).
Now, take every specific action in D&D and give them a page of notes and commentary. You'll see it begins to look very much like its described.
Attacks of Opportunity
Disarm
Sunder
Arcane Spell Prep
Divine Spell Prep
Spontaneous Casters
Metamagic
Grapple
Trip
Overrun
Bull Rush
Spell-like Abilities
Spell Resistance
Sneak Attack
Rage
Bardic Music
Wild Shape
Aquatic Environments
Town Generation
Generating XP/CRs
Exteme Heat/Cold
Status Ailments (Dazed, Blinded, Poisoned, etc)
Blindsight/Blindsense
Turn Undead
Divine Feats
New Skill Uses
etc...