Not entirely because if the lender can bundle these bad mortgages together and sell them it's not really their problem if the borrower can't pay because they've

Not entirely because if the lender can bundle these bad mortgages together and sell them it's not really their problem if the borrower can't pay because they've already extracted a lot of value from the loan by selling it. Being able to sell the debt off to another party removes a lot of the issues a lender faces with bad loans. Now in theory they still want to make all good loans but actually evaluating all the loans in a package is hard to impossible. This can incentivize lenders fudging the numbers on loans because a few bad loans failing is fine (so long as lenders as a whole aren't systematically issuing bad loans of course).