That's because for a monster, their monster type is their class.
But WHY!?
All I'm hearing here* is, at best, a circular argument. "Racial HD are different from other racial traits because they count as levels, and they count as levels because they're different from other racial traits"**
*not from you specifically but from everyone I've discussed this with
**oftentimes it's not even a circular argument, I;ll just get something that's more like "Racial HD are different from other racial traits because they count as levels, and they count as levels because"
And when the rules for playing monsters as PCs came out in Savage Species* and level adjustment was formalized, a PC based on a monster would count both the monster's HD and it's level adjustment for the purposes of level. So a 3rd level cleric minotaur would effectively be 11th level (for the purposes of XP and such): 6 levels/HD of Monstrous Humanoid, a Level Adjustment of +2, and then 3 levels of cleric on top of that.`
And that's another problem I wasn't going to bring up bit now I think you've forced me to. It makes no sense that every monster hit die would burden you with another full level of ECL even if we did count them equivalent to levels, because
1. It results in the preposterous implication the the CR of mirror fight against a creature with the same stats would not be an ECL appropriate encounter
and
2. It's basically asking you to take six levels in commoner and then still be able compete with people who took 6 levels in PC classes...or in adept...or magewright...or expert*
*With the exception of creatures that have outsider racial HD. Outsider racial levels are superior to a level in expert. Although they are still inferior to levels in a PC class, and probably to levels in the NPC spellcasting classes as well
And when the rules for playing monsters as PCs came out in Savage Species* and level adjustment was formalized, a PC based on a monster would count both the monster's HD and it's level adjustment for the purposes of level.
Savage Species also puts the lie to the ideas that 3.x's monsters are built like PCs because a normal PC class would get skill points every level.
A normal PC class would go all the way up to level 20. That's sort of another another problem with RHD as class levels (that sort of touches on what I was saying before) every level is a dead level after a certain point. (And without the savage progressions every level is a dead level period.)
Additionally, if you use Savage Species progressions you throw narrativism and simulationism out the window in favor of pure gamism, because I couldn't tell you with confidence what, if anything, a character partway through a savage progression is meant to represent. Especially since I'm reasonably some of the published ones are for creatures that come into being fully formed and/or metamorphose into what they are out of another mature creature (ie. the drider, the ghoul, the mindflayer, the succubus, etc) so it can't just be an immature member of the race
That's not something you could do in pre-3e D&D without designing a whole new thing around the minotaur
It's also not something that's dependent on bundling all these other stats to hd.
Here's a minotaur character race built using my system in which HD doesn't get you anytging other than HP and immunity to HD gated spells. It is for all practical intents and purposes interchangable with the minotaur built using the official system:
Minotaur characters possess the following racial traits.
*+8 Strength, +4 Constitution, -4 Intelligence (minimum 3), -2 Charisma.
*Large size. -1 penalty to Armor Class, -1 penalty on attack rolls, -4 penalty on Hide checks, +4 bonus on grapple checks, lifting and carrying limits double those of Medium characters.
*Space/Reach: 10 feet/10 feet.
*A minotaur’s base land speed is 30 feet.
*Darkvision out to 60 feet.
*Tough: The minotaur gains 6 extra HD, size d8. This grants 6d8+(con modifier x 6) extra hit points and may alter the effects of certain spells whose effects depend on hit dice (and remember, in this example that's ALL the hit dice do)
*Weapon Proficiency: A minotaur gains martial weapon proficiency with the greataxe and is profocient with all simple weapons.
*+5 natural armor bonus.
*Bonus feats: A minotaur gains Great Fortitude, Power Attack, and Track as bonus feats
*Natural Weapons: Gore (1d8).
*A minotaur has a +2 racial bonus to fortitude saves
*A minotaur has a +5 racial bonus to reflex saves and will saves
*+3 racial bonus to intimidate checks
*+4 racial bonus to search
*+7 racial bonus to listen and spot checks
*Powerful Charge (Ex) A minotaur typically begins a battle by charging at an opponent, lowering its head to bring its mighty horns into play. In addition to the normal benefits and hazards of a charge, this allows the beast to make a single gore attack with a +9 attack bonus that deals 4d6+6 points of damage
*Natural Cunning (Ex) Although minotaurs are not especially intelligent, they possess innate cunning and logical ability. This gives them immunity to maze spells, prevents them from ever becoming lost, and enables them to track enemies. Further, they are never caught flat-footed.
*A minotaur has a +6 racial bonus to attack rolls
*Scent.
*Automatic Languages: Common, Giant. Bonus Languages: Orc, Goblin, Terran.
*Favored Class: Barbarian.
*Level adjustment +8 (although, as I said, I personally would have this be lower, I'm setting it here in the interests of total interchangability)