It's because you blew away the association between your phone and computer. You can't just walk up to any computer to synch your phone, and when you reinstall the OS you start over.
There are a few tools that can rescue your songs. I assume you didn't back up any of your songs, before you reinstalled your OS, right? So your music is on the phone only.
When I had a disk crash a few years ago, I used ipodrip to restore. It worked. Now I do regular backups. Consider doing that.