Bio robots are cheaper to procure, maintain and repurpose, and easier to replace when damaged than actual robots in most cases, making them preferable for menial and dangerous grunt work.
Also there aren't enough "higher level, fulfilling, mental work" jobs to go around, and there will be even fewer once those begin to be automated as well.
Plus, if Amazon unleashed a fleet of robots, every accident would be scrutinized by press and possibly by authorities. With bio robots they can literally cause carnage and no one cares.