It has nothing to do with lore and everything to do with design philosophies.
Players should not be able to create golems.
Developers don't want players to be able to create golems, because it throws all kinds of things out of balance. Golems are quite powerful: they're usually immune to a huge number of things, and many of them can put out reasonable amounts of damage. Furthermore, they ignore most of the restrictions that would normally apply to the players adding combatants on their side: you can only attract so many hirelings with gold or charisma, and you can only command so many undead, but you can have as many golems as you can build and they tend to last for a long time. Together, these mean that if players can have golems then encounters have to somehow be balanced (or balanceable) for both parties that build zero golems and parties that build a lot (hundreds? thousands?) of golems.
The design philosophy demands that there be rules for building golems.
With the exception of 4e, pretty much every edition of D&D has been primarily simulationist; that is, the rules (claim to) provide everything you need to simulate the reality in which the game takes place. In particular, in simulationist games everyone operates by the same simulation rules, so if it's possible for an NPC to do something then it should be possible for an appropriately-built PC to do that thing as well. Thus, if NPCs can build golems (and clearly they can, because golems exist), then there should be rules covering how golems are built that are also available to the PCs.
Therefore, the golem construction rules should be designed to discourage/prevent their use by PCs.
The designers feel compelled to include rules for the construction of golems, but they really don't want the PCs to actually use those rules. Therefore the rules are structured so as to be complex/difficult for PCs to use, since the GM can always use handwaving to provide their NPCs with whatever the construction rules require.