At the risk of sounding a little cynical, I will suggest that there is a profit motive in the move to not applying excess payments to principal unless directly instructed to do so. (money.stackexchange.com)
The incentive is that using excess payments to credit payments due in the future rather than applying it to outstanding principal is more lucrative for the lender. (money.stackexchange.com)
Such excess payments are usually caused by interest earned on premiums paid over the course of the policy's life. (insuranceandestates.com)