Good question.

To explain, it's first necessary to explain what the role of art was prior to the Renaissance.

Imagine a world where writing is something hardly anyone can do, same for reading. In fact you could have a well paid job doing it as a scribe.

Fiction barely exists as a genre. Fairy tales and myths have existed as long as humans but creating new ones is not for painting or writing, that's for plays and minstrels.

Painting and sculpture has a unique quality: it's permanent or nearly so. It's also very difficult. So it's used for important expensive things: telling bible stories in a way which anyone could understand, recording the lives and deeds of the famous, and so on.

Realism wasn't unknown; have a look at the bust of julius caesar in the naples museum. It just wasn't prized because representing things realistically wasn't what art was for: it was for relaying information in abstracted symbols and formalised visual conventions.

If you were an egyptian person or a babylonian person or a medieval person, you could look at your native art and know who was a god, who was a king, who was a hero and what was going on because you'd know the symbols and conventions. It didn't matter that it didn't look like a photo; it wasn't meant to.

Then in the Renaissance a new (sort of) class emerged: non religious people who could afford really good art. And they didn't want religious stuff, they wanted pictures of their wives and families, and classical epics which showed how well educated and tasteful they were. And they wanted them delivered realistically. This stuff was crazy expensive; you wouldn't even believe what the mona Lisa cost.

The rest is art history. For a long time only religion appeared in art; then nobles appeared too, then after a long time common people began to appear. Then damn near anything could be art. And here we are.