I look at my Basic/Expert system - virtually nothing outside of combat mechanics - no skills, no social resolution mechanics, spells are almost entirely combat related with very few utility spells; outside of thief skills, there is virtually nothing codified outside of combat.
This is such utter b.s. that I have to wonder whether you've actually read the books in the last two decades.
Virtually nothing outside of combat mechanics - no skills?: Dwarves have a number of mining and dungeoneering skills; elves have increased ability to find secret doors; halflings have sneaking and hiding skills; there are rules on movement, resting, and encumbrance both in the dungeon, in the wilderness, in the air, and on the ocean, including rules for becoming lost; rules on necessary light in the dark; rules on opening and listening to doors; rules on finding, avoiding, and setting off traps; descriptions of various items of equipment and their uses; rules for spell research and magic item creation; rules on building strongholds and castles; while on B60 there is a few tips and suggestions for handling those things not covered in the rules.
No social resolution mechanics: Page B24 gives a very elegant 2d6 reaction chart that's nicely tied into the charisma attribute and covers pretty much any bargaining, intimidation, or other type scenario nicely. Three-quarters of page B21 is about the hiring and keeping of retainers, again quite tied into the charisma attribute. The Expert rules have an additional two pages on npcs and addresses the subject of followers, stongholds and hide outs.
No utility spells?: Keeping in mind that there were only 6 to 12 spells listed for each level... Cleric's first level... Detect Evil, Detect Magic, Light, Purify Food and Water, Remove Fear, Resist Cold; Cleric's 2nd level... Find Traps, Know Alignment, Resist Fire, Speak With Animals, Cleric's 3rd level... Cure Disease, Locate Object, Cleric's 4th level... Create Water, Speak with Plants, Cleric's 5th Level... Commune, Create Food, Raise Dead; M-U's 1st level... Detect Magic, Floating Disc, Light, Read Languages, Read Magic, Ventriloquism; M-U's 2nd level... Detect Evil, Detect Invisible, ESP, Knock, Levitate, Locate Object, Wizard Lock; M-U's 3rd level... Clairvoyance, Fly, Infravision, Water Breathing; M-U's 4th level... Dimension Door, Growth of Plants, Hallucinatory Terrain, Remove Curse, Wizard Eye; M-U's 5th level... Contact Higher Plane, Pass Wall, Telekinesis, Teleport; M-U's 6th level... Control Weather, Lower Water, Move Earth, Part Water, Projected Image, Reincarnation, Stone to Flesh.
Many rings, potions, and miscellaneous magic items utilitarian in nature as well.
The actual combat rules take up four and a quarter pages of the 64 page Basic rules. That's if you count the one full page combat example and the approximately 3/4 page of rules on morale checks, running away, and surrendering. There are three more pages of combat rules (most of which duplicate the combat rules from the Basic set or extend the tables to higher levels) and another half page on combat at sea. Compare that to the approximately 10 pages of dungeon design and game mastering tips in the Basic set and additional 7 pages of wilderness design and game mastering tips in the Expert set.
Is everything as codified as you'd like? Obviously not. There's only so much you can do in 128 pages.