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