Anything that requires skill eventually gets divided into three parts:
>Shit you can't do because you still suck
>Shit you can maybe do if you work your ass off because you can barely pull it off
>Shit that's easy because it's well within your ability
As you learn your skill (in this case, drawing), all three
of these parts grow. So not only can you do more "fun" stuff like quick scribbles, you also realize how much more you can barely do and how much more you still have no chance at doing. And the easy stuff quickly becomes boring and "not good enough" because your idea of what mastery is continues to grow as well.
That's also why a lot of people who get just a little skill end up stuck on Deviantart drawing the same low-level shit while soaking up praise from their groupies- they dimly see the effort required to improve and just give up. Don't be those losers, embrace the eternal pain.