Why did scholars agree to prohibit selling debt for debt (بيع الكالئ بالكالئ) or credit for credit, despite the facilitation it offers to merchants, especially online merchants?

Selling debt for debt (بيع الكالئ بالكالئ or الدين بالدين) is unanimously prohibited in Islamic law. The reason for its prohibition is the presence of gharar (ambiguity/uncertainty), riba (interest), and potential for disputes. An example of selling debt for debt to the debtor is when a person buys something on credit. When the due date arrives, and the seller does not find what will settle his debt, he says to the buyer: "Sell me this item for another term with an increase," so he sells it to him, and no physical exchange takes place between them. As for selling debt to someone other than the debtor: this is exemplified when a man says to another: "I sell you the twenty mudd (a measure) of wheat that I am owed by so-and-Allah for such-and-such, which you will pay me after a month," or "in exchange for a specific item present." This sale is invalid due to the inability to deliver the sold item.