Because, of course, culture didn't exist prior to the development of copyright...
Commercialized culture didn't. Great art was done almost exclusively by commission for wealthy patrons or religions. All those great Greek statues and buildings? Religious temples and art. All of the famous Byzantine and Medeival art? For churches. The explosion of the new masters in the Renaissance? Paid for by the Catholic Church and the Medici's and other rich Italian families. Great art paid for by the masses by popular demand? It didn't exist. If you made a good living, it's because you found a rich patron. Otherwise, you didn't eat unless you could really wow 'em on the street corner. Even then, most times, artists of all stripes were poor. Patronage made our best art happen.
Good luck getting that system to work now. There's a reason why the "free music" model bands have failed: all of the really talented acts want to, you know, get rich. The copyright system allows them (and authors and filmmakers and playrights, etc) to make a good living. Take copyright away, and the vast majority of the El Cheapo public wouldn't put forth a dime. Bye bye Beatles, Rolling Stones, and Led Zeppelin. Bye bye, Orson Welles, Steven Spielberg, and Peter Jackson. Bye bye, Stephen King, Herman Wouk, Ernest Hemingway.
People want to make money. And the "give it for free and hope good things happen" model doesn't work. If you want to convince Bill Gates or Warren Buffett to pay promising new bands to make records that are available to the public gratis, go for it. I bet they laugh at you, though.