The reason why it can't happen now, is today I loan a bitcoin, tomorrow that bitcoin could be half its value, or double its value... its just too high-risk, and thus there is a pretty big chance of losing money.
All good reasons, but how would you explain that I've been lending bitcoins out for the last year? Use bitfinex, easy, varying return percentages. And the best part is - you lend 1 bitcoin, you always get back 1 bitcoin plus interest, and it's value is... exactly 1 bitcoin! Never half or double of that.