JMHO. Today's car "manufacturers" are really only designers and assemblers. The days when the automaker manufactured (from raw material) the majority of the parts that went into their vehicles are long gone.
Today most automakers manufacture only the frame, body panels and sometimes the engine and transmission, most everything else (Including most of the engine and transmission components) are manufactured by other companies and only assembled by the automaker.
This leaves quality control in the hands of those other manufacturers who are being pressed (very hard) by the automaker to produce the parts they supply at the lowest possible price. I'm certain the automaker spot checks incoming components and sub assemblies, but that is very different from the high level of QC that comes with in-house manufacturing.
Because they have less control, automakers frequently have multiple sources for many items. This also helps explain why in some cases certain vehicles have issues with a particular part (Sub Contractor A) while others do not (Sub Contractor B). Another explanation is a silent design upgrade, where a part is silently changed mid-year. Such was recently brought to light by others on this site with Ford thermostats.