Why cats play too rough?

That's just how cats and kittens play. Their fur protects them from the teeth and claws of their playmates, but to our skin biting and scratching can be very painful. Kittens (and cats) play which is practice for when they are older and need to hunt - this "play" hones their balancing, stalking, co-ordination and pouncing skills. Also, it helps sort out social interactions (what is right and what is not) and hierarchy. Cats, unlike dogs, do not have a strict hierarchy but there will always be a "top cat" and a meeker, milder cat, and all sorts of variations in between.

Cats also do this with humans, and if taught correctly they learn not to be so rough with their human carers and grow up to be well behaved cats.