It’s cheaper to make new batteries than to create a return program and recycle the lithium.
So, no one pays (money) and the batteries go to the landfill.
It’s not necessarily about battery design. It’s that the cells need to be opened, the foil and layers removed and scrapped for lithium. It’s just always going to be a pain to rework than create new.
I was always under the assumption that apple actually recycles them because they want them in good condition, but maybe not? I also wonder if this is a program whose cost is factored in the final price of every iPhone (and therefore a cost that is distributed across the entire customer base), regardless if they are returning their old phones or not.
I was at an open air mall in Borneo (Kutching) and saw all manner of new/used iPhones in official-ish Apple packaging.