Why does the body not recycle red blood cells?
They are recycled, the iron and other components are broken down and then absorbed. Adaptations of the RBC prevent the same RBC being used. They lack a nucleus to make them highly efficient oxygen carriers (pack as much haemoglobin as possible). Without a nucleus and other organelles they're unable to synthesise the stuff they'd need for renewal. There's a lot of wear and tear that occurs as they're constantly flowing in tiny blood vessels and getting damaged or affected by substances in the blood.