I was quite excited when I first landed on this site. I like the idea of sharing resources but what instantly turned me off was that payments were made at random, like the lottery.
Why should I contribute resources only to be told I might get rewarded? 
The point is you should get rewarded and if not you turn your vault off. If many do that the rewards increase in frequency. The network always balances this, so people see what they get paid, make the decision to keep providing resource or not.
A lot like mining, you could run a node for 2 secs and get a block (very unlikely) or 2 years and get a block (more likely, but today still very unlikely).
HTH