Because of the risk on default and loss of the loan. Lenders require private mortgage insurance known as PMI on mortgages with down payments less than 20% from the purchase price.
If lenders paid for mortgage insurance and passed on the cost to borrowers as a higher interest rate, they might have bumped up against those ceilings. If the borrower paid the premium, this potential roadblock is avoided.
Generally, the larger the loan amount, the more risky it is to the lender. Private mortgage insurance is protection for the lender against a borrower defaulting on the loan. If the borrower can’t pay back the loan, the lender has a way to get its money back.