I am not a prolific Shein shopper, I would say at least 80% of my clothes (and other things!) are second-hand. It isn't all about being skint or frugal, but I live alone in an expensive area and (like many other people, including my post-grad educated self!) have been made redundant and had to take a low-paid crap job.
I also like second-hand goods generally, I am environmentally conscious and they make life more interesting.
Having said this, I do agree with you. Shein do some styles that are very different, and the quality isn't going to be brilliant, but it is comparable to a lot of the high street stores. And the quality of some expensive brands isn't great either, for example I have a Burberry coat that I've had around three years but only wear for going out, and only in winter. The lining had gone within the year and its buttons come off regularly! I also have some designer trainers that began showing signs of wear and tear within the year that my £10 (second-hand) Nike ones have not.
Out of interest, I tried to find a duvet cover with a similar design to one of my 'Shein' ones. Jewel coloured with a beautiful bird pattern on it. Nada. Nothing like it from generic shops to high street shops to designer. I was actually willing to spend a lot more on a good one, as I love the one from Shein and it has been discontinued and won't last forever.
It's the same with clothes. I like quirky things, clothes that are a bit different and stand out. Shein is fantastic for this. I am hardly going to traipse around every independent boutique in england looking for a top similar to one I can only find on Shein, I do not have the time for that and I doubt most others do.
And again, as for price v quality with Shein I ordered a gym romper from eBay, £15.99 (the price listed was £6!) and quickly cancelled once I realised they'd misled me with the advertisement. It's a gym romper, it will become covered in oil quickly and the fabric does not need to be fantastic quality for such garments. Almost identical one on Shein, £3.99. I am sorry but, no brainer unless you're very comfortably off.