This problem is not new. In the 1960s I sold menswear at a department store when we had a problem with size variability. Probably a quarter of lines where it matters were not the nominal size. At least the production was usually consistent, a given line was always a size too small or half a size too big.
We were at the mercy of whatever the buyers thought was good and labels according to the actual measurement was never a selection criteria. When a new line came in the boss would open up a range and measure them - this was in the day when shirts were mainly boxed. He would then proclaim the adjustment required for that line to fit. We generally discouraged people from trying on anything but trousers or shoes. I was told it was a hygiene issue but I think it was mainly to avoid having to re-fold and re-box the rejects.
Mostly it seemed the anomalies were too small, the cutter trying to save material we thought. Occasionally they would be too big, I recall one line that was very good value but enormous, the small sizes sold out quickly and we were left with a range of nominal XOS which were the size of small tents. Whenever a really large customer came in he was shown this rack first.
Back to the present. There is no easy way to remedy the problem unless the local retailers make it an issue. If they want to advertise that their garments are correct fit and to ensure their suppliers make them so then the customer wins. If they don't care, as seems much the case, what are we to do? Should Customs create a Division of Size Compliance that turns back badly labelled lines at the dock? Hardly.
It's all too hard and nobody bothers to boycott brands that are wrong, which is why nothing has changed in 50 years. If people were prepared to do anything more than moan there could be change.
As for buying shoes online - you would have to have little regard for your feet.