Another possibility is that heightened focus wasn't an unalloyed good in the EEA.
I can't focus on everything at once, after all; the whole point of focus is that I'm focused on one thing at a time (as in this example, food). If the universe of things other than food that I might notice were I not focused on food-gathering, and thereby be able to exploit or defend against, has a sufficiently high expected value, then relaxing my focus and increasing my peripheral awareness of that universe of things has a high payoff relative to focusing more exclusively on food... at least, when I'm not really hungry.
Yes, that sounds a lot more likely and I'm disappointed that I didn't think of it.
Checklists are powerful, and I don't use them enough. You probably don't, either.
Below are some of my own checklists. Please share your own!
I don't know how to do X.
I don't understand X.
I feel mentally exhausted but can't afford to sleep right now.
I don't want to get out of bed, but I should.
I'm procrastinating on task X.
I'm about to send an email / post a comment of some significance.
(I don't do these ones nearly enough! D'oh!)
I feel sad about not doing a better job at X.
I'm about to make a decision of some significance.
I could go on, but... what are yours? (Now is also a good opportunity to make some checklists for yourself, based on what you think tends to work for you.)