A stock Boston Dynamics Spot robot has a runtime of 90 minutes. Of course the battery can be swapped out, but in a standard work day that would be six enormous batteries. For the sake of argument let's say that the military model has two batteries to run at the same time. That's a minimum of three battery swaps a day and someone who has to carry all of those around. News flash, the operators also need food and water and are going to have that on them anyway. A donkey can also eat and drink pretty much anywhere in the world that's not covered in snow or sand.

Edit: each Spot battery is 5.2kg/~11.5lbs. With two batteries on the robot and four in reserve, that's 20.8kg/46lbs of weight to support a robot that can only carry a 14kg/30lb payload to begin with. Each battery is also the size of a mid-range quadcopter, meaning that someone's job on the team is just dealing with the drone and carrying its luggage. All for a drone that can't smell, can barely hear, and can barely carry anything. Might be able to provide early warning if it happens to spot someone using FLIR from two feet off the ground. Maybe. This is just one tiny slice of the taxpayer's pie being served up to the military industrial complex on a newer, shinier platter with nothing but prop pieces to show for it.