Many reasons why it really is worse than the high street and a little time spent googling why things are so cheap will provide resources to detail this but a taster -
Fabrics and materials used don't have to comply with any basic safety laws, never mind eco/sustainability best practices,
I don't recall the figure but Shein release something like x 3 more new more new lines than the likes of H & M, Zara and Primark,
A total lack of transparency in the company itself, where it's factories are, the outsourcing model they use on whereby they outsource to one who then outsource to another who then do the same ... you get the picture. Anyway this makes it even more difficult to examine worker conditions and find parties accountable when things are wrong,
Their app is incredibly clever (the CEO of either Shein or Temu is a tech guy by trade) in that it throws offer after offer at you which you'd be daft not to use meaning people straight away are buying Stuff They Don't Need.
Then once a customer has worn their £5 top a few times and decided to donate to charity to feel better about buying new again instead of just wearing this top until it's got no life left, the charity struggles to sell it as customers can of course buy new for same price. Hello landfill 
And of course at least our high street chains (who aren't in any perfect are starting to become more accountable) pay tax and generate jobs.