One word: scarcity.

There's this idea that somehow, music used to be a lot better because if you play a classic rock station you'll hear a whole bunch of songs you recognize that are pretty good. Then you turn to whatever Clear Channel is forcing down your throat and the only good thing you hear is that Swedish House Mafia song from like three years ago and you conclude that music sucks now. What everybody misses is that the classic rock station has thirty years' worth of material to fit into a days' worth of programming, while the Top 40 station gets to fill maybe 30% of its programming with stuff older than a year. Objectively, the "classic rock" station has roughly 30 times as much material to draw from. Subjectively, winnowing any 30 tracks down to the one good one is going to give you a better playlist than picking one at random.

Now extrapolate that out to an art history that calls anything from 50 to 3000 years old "classic" and anything newer than 50 years old "modern."

Was there shit in the renaissance? boy howdy. And lots of it was technically exquisite, but no longer matches our modern ideals.

And that's just the stuff that survived... how do you think we'll feel about Jeff Koons in 400 years?

That's only half of it, though. 400 years ago, an artist could make a living by finding a rich person that wanted some self-aggrandizement. Nowadays sketches are things you get done in dorm rooms and state fairs. If you want to get a rich person's money, you have to do something the rich person wants, and rich people generally want things for non-aesthetic reasons. So what you think is rippin' awesome art doesn't matter nearly so much as what someone willing to spend $12m on a stuffed shark thinks is rippin' awesome and we all end up shaking our heads.

There's awesome "modern" art. It tends to get swept aside in reactionary "herp derp I hate Damien Hirst" rants, of which I am guilty of several. But realistically speaking? Those people who are buying stuffed sharks are betting an unholy amount of money that 400 years from now they'll be remembered like the Medicis, as people who fostered the careers of visionary artists destined for immortality. They won't be craven hedge fund managers whose sole stake in the infinite is they headed the breakup of Washington Mutual, they were the ones that put Bubbles in the MoMA. Those who can, do. Those who can't, buy.

Ultimately, it comes down to this: there's ample consensus about what was good and what was bad 400 years ago. There's very little consensus about what's good or bad now. If you're seeing it in a museum, and it's old, lots of people think it's great. If you're seeing it in a museum, and it's new, someone thinks it's great. Scarcity. "Classic" art has been winnowed to the point where the bad shit is gone. "Modern" art is undergoing that process.

"Art is the knack of making something from nothing and selling it."

- Frank Zappa