There is a huge over abundance of second hand clothes. There's enough second hand inventory for everyone in the world multiple times over. I don't think you can blame shoppers for driving prices up because the supply can more than handle it - a large portion of donate clothes are shipped to landfills or dumped in foreign markets.
I blame a combo of greedy capitalistic thinking that prices should be as high as they can get away with, and a skewed perspective of what fair pricing looks like that comes from the artificially low price of fast fashion. If a second hand shop is paying a fair wage for the processing of inventory and that makes the price come out high compared to the original retail, that's a problem with the original price.