With lending money, there's usually a collateral involved. If the borrower defaults, then the lender simply comes after his property, taking more than what was originally lent, purely by the passing of time (which accrues interest).
When you read about student loans for instance, especially higher loans for people who go into law or med schools, how even after those people are employed and making good money, their outstanding amounts barely budge (or even accrue) even after a while. This is unjust, unfair, unethical, and exploitative.
Should everyone sell everything at cost to avoid "exploiting" others?
No one is saying not to make profit, just do it properly. You can rationalize interest as being compensated for risk, but it doesn't make any less exploitative. We have already seen how the economy keeps getting screwed, yet we don't learn.
