The reason the collateral needs to be deposited with a smart contract is because the collateral assets are unseizable. But that defeats the purpose...
Imagine you want to make purchase, but you don't have the funds. You have two options. Option A, you save money first, until you have enough funds. Option B, you borrow money, and save later (while you repay the loan). In economics, option A is called "saving" while option B is called "financing".
Collateral isn't strictly necessary, but it's useful because it reduces credit risk and therefore lowers the cost of borrowing funds (i.e. the interest rate that the borrower is charged). However, notice that if the collateral is equal or exceeds in value the amount borrowed and it is deposited with the lender until the loan is paid off, this is no longer option B, this is option A. This is not financing.
Financing requires that the collateral (if any) is not deposited with the lender until the loan is paid off, or that, if it is, it doesn't cover the whole of the borrowed amount. For financing to work, you need legal procedures that enable the lender to seize the borrower's assets in the event of default. You can't do that with smart contracts, because smart contracts can't seize assets that are in somebody else's wallet. You can do other things for sure, but not financing.