Menes retreating now would mean saving the bulk of his army.
But this obviously had its challenges.
In addition to the apparent river obstacle and loss of face for him, it would also mean his army would start to starve given they had used up their last morsel of grain in the morning.
So considering this, the alternative of trying to match the enemy until dusk and then run away under the cover of the dark might sound like a pretty good idea.
But that then ran the risk of betting that the additional troops would be enough to halt the collapse and enable the army to stay in the fight long enough.
Menes had no way of guaranteeing his reserves would be up to the task.
So, given the two choices, the supreme general began to actually gravitate towards the former choice, thinking a small certain loss was preferable to a risky gigantic one.
As for the concern about starving, well you had to be alive first, then you can worry about food and water.