Why is fingerpicking hard?
The quick answer is that, as you probably guessed, playing that material is difficult! They're playing incredibly complex arrangements that combine a variety of guitar styles, as well as right-hand and left-hand techniques. And they've been refining their chops for long time, so they play fast and with fluidity.
How difficult is fingerpicking?
It is a difficult style to learn, but one way that you can increase your ability to play more is to work on muscle memory in the picking hand. Your fretting hand will pick it up pretty quickly to learn all the chords, but you need to make sure the picking hand isn't being left behind.