SCEA CEO Jack Tretton admitted there was a reason for that: the console isn't ready. According to Tretton, "we’re certainly capable of showing playable game content, but we don’t have a mass-production box that we can bring out and pull out. That’s still in development in terms of final specs and design."
Tretton then claimed that it didn't matter, saying "You certainly look at it when you insert a disc, but for most people, it’s behind a cabinet or on a shelf somewhere and you spend all your time looking at the screen. And we wanted to show people the screen."
""We don’t have a mass-production box that we can bring out and pull out."
Tretton also revealed that the 4K output would only be for photos and video, not for games. He also "hoped" that the price would be lower than the PlayStation 3's $599 USD launch price.