In 1993 rich countries took back 3 in debt repayments for every 1 given in aid. Third World countries found it difficult to pay even the interest on their debts, never mind the original capital. The debt grew and grew. These countries don't earn enough to be able to pay back all their debts. When the IMF offers loans it expects economic reforms to take place.
SAPs usually mean severe cutbacks in public spending.
School fees are introduced and charges are made for visits to health clinics. Companies are privatised, often leading to job losses. Good land is used for crops for export, in order to earn foreign currency and food prices for local people rise. It is people who are already poor who lose out most. The rich countries realise that these debts cannot all be paid.