I feel the same too, the time lapse makes it look so easy.
Another cool thing to notice is that their first attempt at sketching or painting something isn't usually the one they settle with, you can see them slightly repainting an element (it morphs in shape while they're adjusting it).
You can see it more clearly in here (like at 0:25), how the artist constantly uses the warp / free-deformation tools to correct their sketch while working:
Using warp in this way is kinda like 2D sculpting: you start with a sketch that's the best you can do, but then you manipulate/sculpt it until you get it "perfect".