Their muscles aren't strong enough and their heads are kinda heavy so they might fall if their bones aren't strong :)
there bones are bendy sorta like cartilage, also why you never really hear about babies breaking there arms or legs
the muscles in theirs legs aren't developed enough. hope that answers your question :)
There not exactly got there balance and strength to sit up till there older (:
they aren't strong enough to sit up on their own yet.