Because my 6 month old doesn't!
She is a gorgeous, chubby and happy little girl but put her on her mat and she just lies there looking around the room.
She hasn't rolled over yet. She tries to reach out for her toys a little bit but mostly doesn't.
She is no where near being able to sit up!
I also read that by this age they are starting to have a grow a crawling!! DD doesn't even pull herself up on her knees.