Interest is not purely profit. Interest is designed to offset inflation and the other ways you can spend your money. If I owed you $81,000 would you prefer that paid as a lump sum today or paid as a 0% interest loan over 25 years ($3,2400 /yr)?
So in order to make $100,000 today equal to $100,000 in 25 years, we need interest (at least to cover inflation). So for the purposes of this loan $100,000 (at time 0) == $181,000 (at time 25). In addition to the interest, you pay portions of the $100,000 you borrowed. Each payment of $687 is an uneven mix of Interest payments and Principal payments, the ratio changes as the loan approaches time 25.
So if you stop paying halfway through the loan, you may have paid in total $100,000 (combination of interest and principal). But $100,000 (at time 0) is less than $100,000 (at time 25), the lender loses money.