Because Apple says so when they sell you the refurbished unit! They say it will be the same like a new unit from the functional side. And if that underfill (the

Because Apple says so when they sell you the refurbished unit! They say it will be the same like a new unit from the functional side. And if that underfill (the resin/glue referred to in the video) was not important, Apple wouldn't have bothered to put it there for new factory units, consequently increasing their manufacturing cost as well as their refurbishing cost - since removing the black underfill/glue to replace those chips is a nightmare (I know, I do it for a living). On the flip side, underfill provides much more durability and protection from liquid damage (liquid is prevented from slipping under the chip and damaging the balls = contact points = solder joints) and helps the device sustain a lot of abuse. Whoever has children or repairs those devices for a living can tell you the huge amount of abuse they are subject to. So skipping on the underfill process for refurbished units is a "design fault" in my humble opinion. This as well as leaving corroded caps and components on the board. When I fix a MacBook Pro board that was damaged from a liquid spill, I replace every corroded component I can find, even if still functional, to future proof my repair.