I think of it this way. Let us forget about music for a moment and consider language, since we all start to learn that from the moment we hear our first words. By the time we are 5 years old we have a considerable vocabulary, that allows us to express what we want and what we feel - even if it is at a basic level.
We probably haven't learned to read or write yet, or learned anything about grammar and punctuation, but there is a fair chance (assuming we live in a country where education is accessible) that around 5 years old we will start to learn the rudiments of written language. By the time we reach adulthood we have learned thousands of words (I read somewhere that most fluent English speakers use around 6000 words in their everyday vocabulary). We will have also learned to construct these words into meaningful sentences in order to communicate our ideas.
Since, in our chosen language, we all know the words and the theory then why are we not all capable of writing a great novel? Why are we not all capable of creating works of literature that are of the same standard and emotive power as Shakespeare, Dickens, Hemingway or Steinbeck?
Simply, because we need to have hugely powerful ideas to express before we can use our chosen language to express them. The language is simply a tool to do that.
That's not to say you don't have to be skilled in using language. Even if you have the world's most compelling story to tell, if you spoke in a flat monotone with every syllable spoken in a metronomic way you would quickly lose the attention of all but the most committed listener. Likewise, if you spoke rapidly without pauses for breath, without constructing sentences properly and using inappropriate words, then your listeners would rightly think you were just babbling.
To press the point a little further, I think we all know people who can use the simplest language in the most profound and entertaining way, and we all know crashing bores who know all the long words but fail to engage thir listeners.
Now use that as an analogy for what music is trying to express. It should be about the expression of ideas and emotions that are very hard to define in any other way, but you will not be able to do that if you didn't have anything to say in the first place.
Using our analogy, we can all think of people who know all the notes, all the scales, all the modes, all the theory (words, syntax, grammar, punctuation) but whose playing is deadly boring. Likewise we all know music that uses very simple constructs but can make your heart skip a beat.
I'll stop now because I'm rambling, and still haven't said what I wanted to - you see how hard it is?
