Ancient soldiers generally didn't carry knapsacks or rucks to load down with snacks. Sometimes they even fought naked or close to it.Perhaps.... but any seasoned soldier would also have some minimal food with him (like bread and olives and such)...... and there is a difference between not being distracted by food needs during action (hence the need for a strong breakfast to give the men 4 to 6 hours without need for food - though some may have needed to take dumps pretty badly), and standing around all day and not doing anything to get food..... Plus all these slaves and camp followers must have had some use even during battle, not to mention during idle time
What the Ancient Greeks Ate (and How They Ate It) - Greece Is
According to Aristophanes, soldiers likewise ate simple meals, sometimes comprising only cheese and onions.
A few servants aren't going to be able to make/return with a meal sufficient for everyone else, and besides that, in many armies the camp servants fought too, or stayed nearby to help with wounded, or stayed in the camp. Another factor is the army may be some distance from camp, or battle might be looming so while most everyone is hungry they're not thinking of food (even though being hungry negatively effects their fighting abilities).
How many times did you have a big day planned, managed to screw up timing, missed breakfast, and realized that the day is going to suck but you missed it, and without jumping through hoops, you just have to deal with it?
More so, not having breakfast before a battle is more than just about hunger. If you didn't have time to eat, what else was rushed? Was forming up done in a panic? Is equipment all ready? Did leaders have time to even try to come up with even a rudimentary plan? Or was it "Oh crap, the enemy are already forming for battle and most of my army is still asleep. To arms! Hurry up, move your butts and prepare for battle!" An army that didn't have time to eat has bigger issues than just empty bellies and grumbling soldiers...
This stuff happened, its reported by numerous authors over many centuries about numbering nationalities. It wasn't isolated. And I agree, it doesn't make sense. But do you know what else doesn't make sense? Judging whether your army fights a battle or not based largely on watching chickens peck at seeds you give them, then trying to foretell the future based on the movement of those birds. And they did that too!