I think it's not Apple handling the buybacks, it's a third party company that is just really well integrated, and they have determined the best price that keeps them in operation. When I turned in my MBP M1 pro for $950, it was some other company that received it and issued me a bunch of Apple digital gift cards. Anyway, there is a risk to buying back used product - who knows what hidden issues there are in your iphone or how badly you have treated it. The offered prices take into account that risk and the cost of running their operation. If you think you can get a better price on eBay, then sell it there.