The reasons why you shouldn't play with kittens too much are as follows:

Kittens need plenty of rest and sleep to promote their growth and development. Frequently disturbing a kitten and preventing it from getting enough rest can negatively impact its health.
Kittens' bones and muscles are not fully developed, and excessive play may cause them harm.
Kittens' behavior is still developing, and if they play together frequently, they may develop some bad habits, such as biting or excessive scratching.
Kittens' immune systems are not fully developed. Excessive play can lead to fatigue or stress, which may lower their resistance and make them more susceptible to disease.
Therefore, for the health and well-being of kittens, we should provide them with ample space, a quiet environment, and appropriate toys, rather than over-playing them.