Because the loan will require the borrower to provide proof of income and proof of work, and the retirees can't produce this proof when they are unemployed, the pension can only be regarded as the basic guarantee of life by the bank, but not as proof of income, which is not enough to support the monthly payment.
Extended data:
Factors affecting the term of loan to buy a house and mortgage
1: Age of loan applicant
When banks evaluate the repayment period of mortgage loans for borrowers, they first take their age as the basis. Generally speaking, under the premise of meeting the loan conditions, the younger the age, the longer the loan period, and the older the age, the shorter the loan period. Under normal circumstances, "the lender's age+the loan period does not exceed 65 years" is the loan period that the bank can handle for it.
References:
Baidu encyclopedia-loan to buy a house-age influence