PS3's don't run the same platform as PC's, so they'd need to port every game themselves rather than just amass the images already out there. Then they'd have to get them to work correctly with the PS3's hardware(cell is a bitch, remember), especially since no emulator has 100% compatibility. They'd also have to either add new servers or just expand their current ones to handle the additional loads; then maintain them. That's just the technical side.
Before they could even do that, they'd have to figure out what titles to release. That'd take some time and money, they couldn't just look at old sales figures and guess; they need to know what people'd be willing to pay to play now. Then, they'd have to go to the companies and negotiate a new license for their game. That'd take time and lawyers aren't cheap.
Unless they're sure the game is going to sell well, it's no where near the cost in time, money and effort to add every PS1 title to the PSN. The major cost is going to be the licensing, the company's going to want some money, so Sony will have even less money to pay for all of the work they've done. That's why the majority of the releases on the Virtual Console are Nintendo properties, because they don't have to license games they already own. Nintendo is likely losing money on all of their older games, just because they can actually make more money on the more recent, popular games. So, you really have no idea what you're talking about.