The thing is, that the risk you take for example on poloniex isnt as big as the risk you have on loaning btc here,in our home-btctalk.org.
That is why people use the principle "no collateral, no loan" because they are not going to risk their bitcoins just for a small return. They need something with value, to gain back the money loaned to them in case of scam.

I would treat poloniex more like a steady investment, where you can get some % of return from money invested pretty much without any risk, they just cover you up.