In the UK charity shops are largely exempt from business rates, so they have a trading advantage over 'normal' shops. In other countries it might not be economic to run charity shops (or 'goodwill stores' as they're known on the other side of the Atlantic).
However, it might be wrong to assume that they don't exist at all. A tourist visiting London would probably never see a charity shop, simply because most of them are located well away from the usual tourist hotspots. It might well be that visitors to Rome, Paris, Madrid, etc simply don't come across charity shops because they're not visiting the parts of those cities where the shops are located.
Chris
PS: You're not the first to ask this question
http://www.theanswerbank.co.uk/Shopping/Questi on451380.html