Even if the interest rate is 7% per annum, why would anybody take that kind of risk with their money.
You are basically giving out your money to someone you don't know and hoping to get 7% more in return at the end of 1 year.
You know how good hackers are at hacking such exchanges and services right. I am sure I would never take the risk of investing my money just for a mere 7%.
I would rather hold it in my wallet and hope BTC to increase 7% more than it would normally do.