When mammals are young we all love playing. This is true for humans, dogs, elephants, primates, cats and so on.
But play ends up disappearing as the mammal gets older. Compare a playful baby elephant to an adult elephant. The baby elephant loves playing. Not just with other elephants but with humans and dogs and so on. But older elephants are more likely to watch and less likely to play.
This is why your 1 year old cat loves playing while your 17 year old cat doesn’t.
Play is a semi-structured way of learning important skills! Evolution really just has to make it engaging enough for you to learn the skills, not much of a reason for it to persist after that.
I am clearly speaking from the perspective of evolutionary pressures. And you're right, there's very little evolutionary pressure on working out, staying fit, staying happy, etc. etc. beyond the bare minimum required to ensure propagation.
There's a theory that the domestication of dogs selected for the retention of juvenile behavioral traits into adulthood (non-suspiciousness, playfulness), vs wolves who do not retain those traits.
It's even thought that the first domesticated dogs pre-domesticated themselves to an extent because the less suspicious wolves were able to use human settlements' trash middens as a food source.
All mammalian species have something in common.
When mammals are young we all love playing. This is true for humans, dogs, elephants, primates, cats and so on.
But play ends up disappearing as the mammal gets older. Compare a playful baby elephant to an adult elephant. The baby elephant loves playing. Not just with other elephants but with humans and dogs and so on. But older elephants are more likely to watch and less likely to play.
This is why your 1 year old cat loves playing while your 17 year old cat doesn’t.
It’s a very interesting phenomenon