The rise of video and film art appears irresistible. The Turner prize has just been given to a video for the second year in a row.

Yet in spite of the successes of Laure Prouvost and Elizabeth Price, the triumph of video art is an illusion. It is not a stable, enduring art form; it may not even be an art form at all. It is in reality an experimental space at the margins of a much bigger culture of the moving image – a place for talented film-makers to mess around with a freedom they could never enjoy in commercial cinema or mainstream television, but which the true artists among them hunger to apply in those bigger, more important arenas.

For it turns out that video art is just a training ground that can prepare you to make proper films.

The proof can be seen in the achievements of a previous Turner prize winner: Steve McQueen. In the 1990s McQueen was making bold and powerful films for galleries; in 1999, he was awarded the Turner prize; ever since, he has continued to show acclaimed art.

Reading on mobile? Watch a clip from Steve McQueen's Deadpan (1997) here

And yet all this seems irrelevant compared with his emergence as a "real" film director. His first feature, Hunger, was an extraordinary portrait of IRA hunger striker Bobby Sands. He went on to portray sex addiction . Now he is in the running for the Oscars with 12 Years a Slave. This film is a phenomenon. "It was the most powerful movie I've ever seen in my life," is a typical response to it in New York.

Plainly, the art world has merely been a kind of postgraduate film college for McQueen – and as such it is the best place a serious film-maker can experiment in the 21st century. But that is all video art is: a bit of messing about. The important films are the ones with scripts, actors, and stories.

The moving image has gripped the human imagination for more than a century. It was always inevitable that one day art galleries would embrace it, too. Video art however has gone through so many changes in its brief history that it does not have much aesthetic coherence at all: does an installation by Gary Hill from the early 1990s belong in the same category as a more recent work by Cory Arcangel? Are they doing the same thing? In the end, moving-image art is just a catchall term for the many ways artists have fun with cameras, screens and software.

What all such arty activities have in common is that they exist in the shadow of the much bigger phenomena that are cinema and television. Artists may like to think they are in opposition to the movie mainstream, but the facts show how keen they are to cross over into real film-making.

McQueen is not alone. Sam Taylor-Wood, now Taylor-Johnson, has moved from the gallery world to commercial cinema. She's making the film of Fifty Shades of Grey. Douglas Gordon too leapt over the wall when he collaborated on the (admittedly very arty) football film Zidane: A 21st Century Portrait.

There is no doubt the art world is breeding some skilled film-makers – and equally no doubt they are eager to prove themselves among the big girls and boys over in the hills of Hollywood.

12 Years a Slave is both the greatest triumph of video art – and proof of its cultural irrelevance. Who'd trade an Oscar for the Turner prize?

Reading on mobile? Watch the trailer for 12 Years a Slave here