I think that manufacturers of tech products test their products only with a few standard configurations - but in reality there are too many possible combinations of different configurations:
Take a bluetooth mouse for example. Generally, it connects to a computer and it works. Now imagine that you have a different configuration - a logicboard in your laptop that has not been tested by the manucacturer of the mouse or an obscure model of the bluetooth reciever, that also hasn't been tested to work with that mouse. Your mouse works well in the beginning, but disconnects at random times. You can't pinpoint the issue, and when you are looking for help online, nobody seems to have the same problems with that mouse.
In this case, said mouse sucks, because it doesn't function reliably. A different person with a different configuration of their computer (different logicboard, different model of the bluetooth unit) might have no problems at all with the same mouse.