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.