I'd assume it's because while most spells don't scale with a 1.0 ratio with AP, every champion has at least one thing that scales with a 1.0 AD ratio, namely their auto-attack. Which also happens to be on a much shorter cooldown than all spells.
Essentially, you have to remember that attack damage, attack speed and crits all scale multiplicatively with each other - when you have a lot of attack damage, increasing attack damage by a bit will net you less of a damage-per-second increase than increasing attack speed by a bit would; if you have a lot of attack speed, you prefer to increase your attack damage, and so on.
I'm not sure I should be talking - I mean, I only have about 600 games played total so far, none of them ranked - but I'd say Infinity Edge->Phantom Dancer->Bloodthirster->Black Cleaver is just about optimal on most regular AD carries (with defensive items as needed in between, which means it's quite possible you can't get all four), as far as the progression of damage items goes. Or at least, it's not completely off on pretty much all of them.
I usually go Doran's Blade -> Boots (Berserker's or Mercury's, depending on the enemy team) -> Infinity Edge, with a Vampiric Sceptre somewhere in between while building it, for lane sustain -> Phantom Dancer -> Banshee's Veil -> Bloodthirster -> Black Cleaver or another defensive item, on more classic AD carries like Ashe or Caitlyn (with increasingly different builds the more atypical a carry it is).
EDIT: Ooof, Kinkou'd so hard.
@Daverin: You wrote in the previous thread that you sent me a friend-request on EU-W, but I never received any.