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.

Seriously, https://www.youtube.com/watch?v=9fa9lVwHHqg

Human 'soldiers' are really only going to be useful in the near future for infiltrating and disrupting social systems, not for dispensing physical violence.

I think you are both way off, we can't get a robot to flip a burger[0] it's going to be a long time for a robot to replace a special forces solider. They aren't just dumb manpower, we are talking a strategist, weapons specialist, paratrooper, negotiator plus a dozen other roles all in a 6 foot tall, 200 pound package that can go into a country and forage off the land for months while waiting for the perfect opportunity to complete it's mission.

[0] I know you can find something on Youtube but if it really worked at scale McDonalds would be rolling them out yesterday.

People said the same thing on the advent of the Air Force. It turns out there's a lot more involved in winning a war than murdering people.

That can be taken to its logical conclusion, like that classic Star Trek Episode:

Why bother building killer robots and drones when you can simulate the outcome of a battle without having to fight it in meat space.

Whoever loses has to line up at the suicide booths.

https://en.wikipedia.org/wiki/A_Taste_of_Armageddon