Scouts are specialized functions. You program the scout to do that and nothing else. This produces a robot that is very poorly suited to do anything else.
So why not make a specialized battle bot? Or rather, dozens of kinds? Because a soldier in battle may do all sorts of things. One might be artillery, one infantry, one sniper. It is not feasible to program a bot to do all of these, but while a human can specialize, it is feasible for a soldier to have a smattering of skills from other specialties, and also to pick them up on the fly. (A sailor at Pearl Harbor manned one of the guns despite having training as a cook.) Consequently, an army of soldiers is much smaller than one of bots.
Furthermore, soldiers have better judgment than bots. If a scout is destroyed through bad judgment, it's just a cheap bot. Soldier bots will lose you the battle.
Not to mention the horrific incident where bots were programmed to distinguish civilians from soldiers by their uniform. When they were attacked by civilians, they not only killed the civilians attacking (who were, technically, war criminals) but many others who were wearing the same thing. No one will trust a bot to learn on the battlefield what a uniform is. BUT that means an opposing force can defeat them by disguising that they are the opposing force. Pattern matching makes it impossible for a bot to reliably distinguish.