Animals are pretty resilient, compared to robots. Most animals aren't anywhere near as picky about their fuel source or when they receive it as robots are, especially over weeks-length periods of time. Minor-damage is self-servicable and self-repairing. Even the smartest robots are barely at the intelligence level of the dumbest animals. And some of the more intelligent animals are self-adapting to new, out-of-spec scenarios.
There are tradeoffs, and in many situations machines have a clear advantage over biological organisms.
In aviation and space, there are serious issues in keeping biological organisms alive due to oxygen levels, pressure changes, and G forces. A purely autonomous system would be able to be more maneuverable in more environments than any system that also needs to keep a meat bag alive.
The other thing machines are great at is patience. Land mines can remain operational for decades, just waiting for their opportunity to detonate. Biological organisms have much higher passive maintenance costs.
