To me, chess is very much like music.
In music one can study theory to a very great depth. Every musician has met at least one or two people who have just encyclopedic knowledge of theory and musicology. They know their stuff backwards and forwards. But they really would be of zero use to a gigging band because they just aren't that good at playing their instrument.
And every musician has met the opposite. Some guy who just does his thing but knows nothing about theory or musicology but who can outplay just about anyone around.
Chess is very similar. While playing chess and knowing chess information (be it opening theory or tactical skills) are intimately related, as are music theory and performance, knowing when to do what is something you can't learn through study. Rather, you have to learn it by doing it.
That's was Botvinnik's great insight when he said that chess could not be taught, it can only be learned.
As a lifelong student of various things--literature, chess, piano, college courses, and more--I've often thought about why it is so hard to study to get better at chess.
Now, of course it's easy to study chess; just solve tactical problems until the cows come home, master essential endings, get a thorough working knowledge of some book openings and opening principles, read over and analyze your own and master games, and more. But so often doing those things seems to result in only marginal improvement over the board.
The reason, as I see it, has several facets. First, you're never just objectively better. You're always playing an opponent, whether human or computer, and your success must be relative--always dependent on the performance of your adversary. A weightlifter, for instance, trains to lift a certain weight, masters the lift, and can do it in every competition. If he loses, it's only because his adversary could lift a heavier weight going in. He's never surprised by some six year old with a secret lifting technique, nor does he ever lose because he made a subtle oversight and thought he was lifting 200 pounds when it was actually 300.
Second, situations arise in every game that you've never studied for and that you've never seen. You didn't prepare for them because they've just never come up. Then, no matter how much or what you've studied, you're on your own again and have to "create" your way through at the board. This never happens to, say, pianists. If playing a Chopin etude, no matter how complicated, the pianist is never faced with a few fantastically intricate measures he didn't know were in the piece, forcing him to read and play them at sight as soon as he comes across them.
Of course I'm not saying that chess is impossible to study for. That's not the title of the thread. Just that it's so HARD to study to get good because of all those unforseen and unprepared for elements that get in our way. It's three steps forward and two steps back--always. But we do get there eventually.