Someone explain why people dont buy phantom dancer

Ya, shiv has good early game power, with it being very cheap and the 100 magic damage is good early game, but late game, that 100 magic damage proc will be negligible, and the stats are far inferior. when we hit the 40 min mark, and my adc is full build and STILL has shiv, i dont know what to say besides TRADE IT OUT. for reference, heres the difference if you upgrade to phantom dancer +15% crit chance +10% attack speed -1%move speed -magic damage proc +move through units (more useful than you think. how many times do the creeps get in your way in a single game?)