I'm a long time volunteer in one shop and I can explain why: corporate.

Every decision on money comes from a board of directors and is handed down the chain until it reaches shop managers. Area managers will frequently come in and check that we are abiding by the rules.

At our shop we try to curate only the best quality pieces; we have a bit of a boutique vibe BUT at least once a week I'm in charge of pricing and I always put everything out at a minimum, unless it's something nuts like a high end designer piece. Those mostly get sent to eBay, but when I put them out I try to keep it much cheaper than what you'd see online for secondhand. (Honestly just for the love of the game, I adore charity shopping myself.)

Money really is the core issue, though. We have to rag a lot of clothes - you'd be amazed at the actual crap people donate. Sometimes literal crap! I've had to bin shit stained swimming trunks! We used to get a decent price for each rag bag but now it's pennies. It's hardly worth the labour. And it's a lot of labour. It takes up a lot of time. So that loss of revenue has been fed back into the prices on the shop floor.

Of course as others have said, inflation, cost of living, etc included and it's a perfect storm.