This question is often easily answered with, “Well they don’t make them like they used to…”
And this is essentially true. You only have to look at the fittings at the top of the picture and compare them to the antique items underneath, to understand that the old parts were heavily over-engineered to last the ravages of time and abuse by hand.
It’s not just the size of the threads and the shafts that are dissimilar, the galleries where the water travels through the body are less than half the size too. So when an old tap is replaced with a modern Chinese piece, the owner will always notice that the amount of water has reduced too, taking TWICE as long to fill the bath.
Both of the tap parts above are from bath mixers, but the modern import only uses 1/2″ washers. These are the standard fitment to basin taps, so of course the tap will be strangling the water flow.
Using less materials is a very green initiative, but not in-building failure. You can see that the old fashioned shaft has been replaced by 3 parts now and each of these raises the chance of something going wrong.
Another saying goes, “If it ain’t broke, don’t try to fix it”. But we add to that…….  if it is, get it refurbished, not replaced!