Mechanical devices don't repair themselves. Living bodies do. I think you underestimate the highly optimized and resilient nature of a meat-sack body.
Only because we haven't bothered building any that do. A cynical take would be that it would be bad for the economy.
This will end up in a goalpost-shifting debate where the extent of self-repair capabilities necessary to qualify as "self-repairing" is argued. I'd say that a fault-tolerant system is at least 50% of the way there.