I used to shop in Primark when the kids were little/younger because they grew out of clothes within a couple of months, so they would outgrow them before they wore out. IYSWIM. So I saw no point in paying loads of £££ for something that wouldn't fit them in 8 weeks time! But I never shop there now. Most of the stuff is poorly made, cheap tat that goes to shit after half a dozen washes or a dozen wears/uses. Shoes, jewellery, bags etc, often break after a dozen or so uses.
As has been said, it is false economy. I am not a snob, but the stuff really is shite. I would rather buy clothes from Tesco F & F or Asda George to be honest, than Primark. I don't get how it's so big and successful. Their products are wank. All the ones I have bought anyway!
As you say OP, the clothes lose their shape and look like shit within a few washes. The pyjamas and underwear present as OK/decent quality, but they don't last (IME and that of everyone I know...) Some people have mentioned other shops that are poor too, like H & M, and I agree, they are shit too.
Maybe they do have some stuff that lasts OK, (some people seem to be reporting some things that are OK,) but I would be lying if I said I had ever found a single thing from Primark that has lasted long-term/indefinitely. And I have bought quite a few things from there over the years. As I said, especially when the kids were younger (the noughties,)
I have jumpers and cardigans that I got from M & S, and C & A, and Debenhams, and BHS in the noughties and 1990s that I still wear occasionally, and they look like they did when I bought them. (Even a small handful from the 1980s.) They are the same colour, and the same shape. You don't get that now. Clothes do NOT last and wear like they used to... Fair play I did pay £15, around 25 to 38 years ago, (which is about £40 to £48 in today's money,) but these garments have last 2 generations almost.
It is ridiculous to call someone a snob because they prefer to not buy cheap shit that will wear out in a few washes/uses. If you buy stuff that lasts a lot less time - you will end up spending more in the long run. Not rocket science is it?!