Why is it forbidden for the borrower to loan to the lender, despite the borrower being the sole party harmed, and are there any Shar'i texts that clarify this?

Usury (riba) is one of the gravest major sins and destructive acts, incurring damnation and obliterating blessings. It is forbidden for both the lender and the borrower, based on the hadith: "The Messenger of Allah (peace and blessings of Allah be upon him) cursed the one who consumes riba, the one who feeds it to others, its scribe, and its two witnesses, and he said: 'They are all equal [in sin].'" The one who takes and the one who gives are equal in sin.