Donor countries hope that by giving a lump sum of money to countries, they can fix multiple things and it will ensure that those fixes are effective. If only small amounts are given that don't lead to a long term fix, that means more money is given in the long run. If they can fix their problems by donating a large amount, it could save them money from future donations, make them look better by being generous, and maybe even make money from trade if the country grows.