Andre1000 is right, but there are two more reasons.
1. Posting content on LPSG is risky given that you can get banned if someone gets butthurt and files a DMCA claim. Trades are done by PM, so it's usually a way to get out of the public thread.
2. If someone wants something I have, I'm usually happy to share by PM, but not if the person's account shows that he doesn't share (or even talk!) himself. You'd be surprised how many PM's I get asking to "share" from accounts that are one, two, five years old and have zero-to-ten lifetime posts. Since I'm not the Fairy Pornmother for these leeches, I propose a trade instead, and in almost every case, the other person doesn't respond.