That's a very insightful and peculiar observation.
To riff from your thought, play is evidence of endogenous programming for exogenous adaptation that expresses (works out) reactions ahead of the need (fitness). Play is behavioral echos from previous generations.
Why don't fitness traits just manifest precisely when needed? Why any rehearsal via play?
A picture forms in my mind of a flying insect emerging from a chrysalis— or a fledgling— the stretching of wings. Why? The exercise action is a second order effect, or beyond, of the expression of the intrinsic trait which precedes its cause.
A true notion of time is necessitated, because without it, the explanation of play lacks a substrate (dimension) for meaning.
But a notion on time only embelishes the puzzle of moments of reaction before the need. Why does the genetic program run a test under safe conditions; how does it know?
Where is the reality?
With these thoughts time takes on a freaky character: a manifestation of a nervous system.
Just as play is an expression of reaction before the need, there is the expression after the need: thought.
Memory runs forwards as well as backwards. Reversibility goes out the window: it's only a physicists longing for symmetry.
Play at a full ideation usurps all meaning from reality: reality is carried by the organism. Everything that seems substantial about the environment is a quavering field.
Holes in space called stars pour energy across a membrane (yin-yang) with life's reality manifesting at a crossing (+).
Play is the pre-doing of what has already been done.
