In some ways those constraints are what make games mentally satisfying, because they relieve us of what existentialists called "the anxiety of freedom." By giving us obvious, well-defined goals, they save us from having to define success; and with points, leaderboards, heads-up displays, indicators, badges, etc., they tell us exactly when we've achieved it.

Humans crave that kind of structure, probably because we get so little of it in real life. It's a lot harder to say whether you "have a healthy romantic relationship" or "are making a lasting contribution to something bigger than yourself" than that you've "lined up the yellow gemstones," "scored more points than the other team in twenty minutes," or "collected forty pounds of silver."