A bag will come in full of items. A small subset of that bag will be pulled out to go on shelves. The majority of the bag will be sent to "salvage" (trash/recycling).

Some folks have a curated selection of items they bring in that mostly get accepted, but usually the folks doing that just take it to a real consignment shop, or sell it on something like facebook marketplace.

Generally - a large portion of the donations are exactly this situation: used items in a big trashbag that come in after a move (or eviction [or death]).

They go in the trash. Where the owner probably should have put them a while back. I also don't like how much churn exists in our modern consumption economy - but it's not doing favors to pretend it doesn't exist. Much of our "stuff" is low cost, semi-consumable items that will end up trashed - by design. I don't like it either, but saying "Donate it" is like pretending recycling is going to solve plastic pollution.