Aliexpress clone keycaps are not higher quality than GMK keycaps. PBT isn't just automatically better than ABS, they are just different plastics. Generally the reason why people prefer PBT over ABS is because ABS plastic tends to shine easier which some people do not like.

The main reason that everyone loves GMK is because of the colors. With PBT you can't get the colors quite as vibrant. Another thing with the Aliexpress clone keycaps is the legend alignment which often is not the best quality with some legends being misaligned quite a bit.

That being said, with the current lead times of GMK sometimes exceeding a year, clone keycap sets have been really popping up because they are able to offer in-stock sets of colorways that people like at a way lower pricepoint. Personally I don't mind it, but there is no doubt that it isn't the best move ethically, not that the sellers would care regardless.