Most of the stuff is hardly worn or doesn’t fit anymore
Take it to a charity shop instead where they can sell it on to make money to help others. eBay (and probably all these places popping up that will but your old clothes at ?£/kg) has really reduced the ammount of good quality stuff going to charity shops as people would rather make 50p profit for their old Bros t-shirt or Right Said Fred single than let a charity shop get £1 for it 🙁
The majority of the stuff that goes in charity clothing bins, as stated above, goes to rag men who pay per kilo and ship it off to Africa. Although I did see one the other day, either Barnados or Oxfam I think, that said the items do get distributed to their shops. If you take it to a shop and they deem it unsaleable (is that a word?) they will then bundle it up and at that point it goesto the ragman – there is more profit selling it through teh shops though.