Why doesn't the lendee just offer the moneypak/whatever as collateral?
Because thats not safe. They could give the code to the lender and the lender just redeem it and not send btc. Its always possible though. You could simply just give them the moneypak code but it would have to be someone with great trust. With this method you could set up a third party to send both items to and have a 100% guarantee that nothing could go wrong. If they fail to repay the loan then simply cash on their collateral coins. But it's just an idea
Edit: or the collateralcoins website could offer to be the third party making everything even simpler