Let's discuss a specific example. Let's say I have 10,000 BTC and lend out half of it at an average monthly interest rate of 10%, profit per month is 500 BTC.
There is more demand for loans so I have a choice, I can lend out more of my own BTC and eliminate my operating capital buffer or I can take on deposits.
Let's say I am able to get people to deposit 5,000 BTC in two month CDs at an average monthly interest rate of 6%. I lend it out at the average monthly interest rate of 10%, additional profit per month is 500-300=200 BTC.
So, I have increased my profit by 200 per month and I still have my 5,000 BTC operating capital.
With this example in mind please describe your concern.
No sane person actually believes that there exists some massive supply of ultra-low-risk, ultra-high-interest loans. And no sane person with the wisdom to exploit such a pipeline to financial success (assuming it exists, which it doesn't) would also be so stupid as to borrow money at 100% APR. You can treat other people like idiots but I'm not falling for it.
6%/month is slightly more than 1.5%/week. As Gavin put it:
It is impossible to get risk-free 1.5% weekly interest, I don't care how good you are at "playing the market" or "choosing who to lend to."