I really dislike reaching for these sorts of evolutionary "explanations". They're typically unconvincing ad hoc just-so stories with no real attempt at systemat

I really dislike reaching for these sorts of evolutionary "explanations". They're typically unconvincing ad hoc just-so stories with no real attempt at systematically establishing the truth of the various mutually contradictory claims. Evolutionary psychology is notorious in this regard. Instead, I will only add that distractability is emotional and moral in nature. When I feel uncomfortable, I will be tempted to switch to something else to relieve the discomfort. The inability to engage in self-denial is one reason for indulging the temptation (which is why fasting and abstinence are great ways to engage in self-denial and strengthen one's self-mastery; you can always spot a person's weakness by the passions they overindulge and which seem to control them). Historically, this inability to do without pleasure was called effeminacy and the inability to bear discomfort was called delicacy (the Greek μαλακία has been translated into either, I think). Sustained focus is toil and toil is uncomfortable and requires both enduring discomfort and relinquishing pleasure. Another reason may be related to mental health. Distractions and diversions become especially attractive when under duress.