Actually, beginners get told all the time. But they (and when we were beginners, we too...) tune that out. Who wants to know that it takes time to get good? Why the nice people from Nikon and Canon and Sony all sing from the same songbook - get our gear and presto! Instant works of art!
It also takes time to cultivate a sense of what is really good, and what is just ok. It takes time and training to be able to discern the subleties of taste, of smell, of sound, of vision. Some people get there faster, some slower, and some are unfortunately tone-deaf and image-blind.
I think the time needed to cultivate that sensibility and the ability, is what Glass refers to, and also what Gladwell talk about. It takes time and effort. And generally beginners don't want to know that. So it's not that they are not told... It's that they don't hear.