because in the past,in MacBooks particularly,Apple used mix of LG and Samsung displays and as expected,Samsung ones were superior,people went as far as digging displays serial numbers to identify wether it’s an LG or Samsung display on their machine they’d bought.
There are numerous threads regarding it here.
The point is,Samsung (and Sharp and Japan Display) have always been better than LG.
Apple’s quality test is irrelevant.LG will pass that,but their displays are not as good as Samsung and other mentioned ones.
iPhone X had Samsung OLED display.