Why can't a baby stand up straight yet?

Because when you are first born as you can imagine you have never done any sort of exercise or activity if you stop doing stuff for 2 month just sit watch tv you would find it hard to walk as well but the reall answer is muscle they have yet to gain strong enough hip bones knee bones and over all muscle to support there weight over time these muscles will develope over really unnatural causes like just playing the standing game thing and in time you will wish they never devoleped those leg muscles