Why do some lenders require borrowers to secure credit? to prevent defaults to guarantee full repayment to avoid any losses to reduce risk

I believe the answer is:  To prevent defaults

In credits, defaults refers to the inability to either paid your credit or interest when the time is due. As the number of defaults increase, it would be much harder for the borrower do propose a similar credit in the future because their credibility is dropped.

Do you have options for this answer