Because they give you every stat you want as the ADC. Let's look at what stats you want:

Damage
Attack speed
Crit
Armor penetration
Lifesteal
Movement speed (though not as important as the above).

The items mentioned give all of these stats, and distributes them well. Attack damage carrys do not want cooldown reduction, mana, etc. These are the only stats they are really interested in, and they give all these stats without a gimmicky passive (like Hurricane).

If you want ADCs to build differently for each game, you need to look into changing these 4 items completely, nerfing them doesn't help unless you nerf them to the point where no one builds them. The stats provided fit what's required too perfectly.

Except they never intended for that not to be the go-to single target damage build. they want AD carries to be able to spec into other things if they feel the need to like on-hit kog with hurricane, MF ult with black cleaver or BotRK on anyone for kiting.

Oh, I'm sorry. I didn't know variety was possible in League. I'll go back to the same build every game.
The same build.
Every..................single............game.

or even better, 4 black cleavers

Phantom Dancer? not so much, lots of people build statikk shiv instead now.

Hurricane Ashe because that constant AoE slow ftw. Or Stattik Shiv Ashe because cute combinations.

But seriously, Mr. Weasel summed it up - you could go standard build for high single-target dps, or you could do branch builds to build into what your team needs.

Working as intended.

Or you could be kogmaw and use a real build

I don't mind so much that the build exists, but what bothers me still is that it's only 4 items... still leaving room for another item to be added.

It would have legitimately helped if they would have done something like split IE into two items, so that to get that kinda potential you'd actually have to commit all your slots (besides boots) to it. Otherwise, if you want something like GA/survivability? You have to actually make a sacrifice in your build (sacrifices like every single other class has to make)