As an experiment: on land crouch down low so your back hand touches the ground, feet like they are on a surfboard, looking forwards. Notice how your head will be centred over your body and you are bending at the knees, hips are low. You are probably quite stable. Now stand up - you probably will (should!) go straight up ie not leaning forward then back or anything. Your weight stays pretty evenly balanced across the heel and toe. This is goodness!
Now (on land again) stand tall and then move so your head drops about 1.5ft from normal. How do you do this - do you bend at the waist and your head goes forward? How stable do you feel? This is badness!
In any case, by staying low after the pop up you put your body and your balance in a much better position, then you rise up (and go back down) as you need to - and you need to go up when you need to turn. Whereas if you start off high you tend to go lower by bending at the waist, head goes forward, butt goes back to balance, its all not very aesthetically pleasing (let alone effective)
As waikikichan says, pop up and turning are different actions. But get the pop up right and the next stage flows easier.
Of course, you can 'pig dog' along the face of the wave staying low the whole time if that is appropriate.
