Great point! I'll first admit that I am one of those WD element shuckers and have been using them to fill my synology unit for years now
That aside, I'll attempt at responding to your point. Harddrives all cause vibration and those vibrations add up if you attempt to mount a bunch of them on a shelf or rack. If you put enough of them together, their vibrations will eventually add up to a harmonic wave shaking the entire rack and dramatically increasing the failure rate of your drives (so a lot of antivibration work will be needed). the failure point for consumer drives is around 5-10 drives, whereas with purpose built internal drives that failure point tend to be a lot higher with built in vibration resistant mechanisms inside of the drives.