Why it keeps getting harder

Thomas Mann once defined a writer as "a person for whom writing is more difficult than it is for other people." And a corollary -- the better you get at writing, the harder it becomes. It pretty much has to. Your standards get higher -- you demand more of yourself. And as a writer, you're in competition with everyone else who's writing and everyone who's ever written. In a sense, that's a fairly steady level of competition, but in a more significant sense, it's not, because you recognize more of your competition's salient points as you get more experience.
And in an even more significant sense, it gets wildly difficult, because your main competition is always yourself, and as you get better, it means your principal competition gets stiffer.