If people knew how hard your team worked on that production they would come see it.
If people knew the care and thought you put into every painting they would buy it at almost any price point.
If people knew how much you considered every word in that book, they would gladly come to the book signing and buy multiple copies.
But they don't know.
That is, in a nutshell, why you need effective marketing.
We are in a world with endless options. More importantly, we have an endless number of good options. So we need some way to help us choose. We need signals, we need something that helps us figure out that "this is for me."
That's all good marketing is. You use all the tools available to tell a story. That story connects with a specific group. They say "I want this." The rest say "not interested".
People need to know the good things about your art or they will not pick it. That's why you need marketing.