Assuming you mean the First World War, this was not always the case; some international Belgian relief vessels did indeed get sunk, on occasion, though you're right that it was rare.

Nevertheless, they tended to be spared for two reasons:

It was thought (correctly) that sinking them on a wide scale would be a disaster in terms of Germany's international image. These were ships, frequently of neutral powers, being used for missions of mercy; in spite of the policies of unrestricted U-Boat warfare that existed at various points during the war, the gains involved in sinking such ships were minimal compared to the public relations nightmare that such predations might conceivably cause.

A more important reason was one which saw such relief operations tentatively opposed by certain Entente statesmen, including Winston Churchill. Belgium was a wealthy country, if small, but it was not rich in agriculture; at the war's outset, only a quarter of the food necessary to keep the country functioning could actually be grown in Belgium itself, with the rest having to be obtained via import from friendly powers.

With the German occupation having turned all of the Belgian economy (basically) towards supporting the occupying army, the population was at a very real risk of starvation. Every relief ship that made its way through was a batch of supplies that the Germans did not have to provide to their Belgian wards themselves. The Entente thinkers I mentioned above were opposed to this because they agreed with the German assessment of likely consequences: mass famine leading to civil disobedience, riots and uprisings, even as the German army's own supplies and morale dwindled week by week. While the Entente powers were certainly in favour of assisting the Belgian people were possible, it was also thought that doing it in this way was likely to prolong the occupation -- and the war -- rather than otherwise.

Whether it did or not is more than I can say, but it remains the case that some 11.4 billion pounds of food was shipped to Belgium between 1914 and 1919, including nearly 700 million pounds of flour.

The ship was the SS Imo a Norwegian flagged ship. Norway was neutral and so could expect not to be troubled so long as it was not carrying weapons. The ship had the words "Belgian Relief" painted in letters about ten feet tall down the sides. The ship was chartered by the Commission for Relief in Belgium to deliver supplies (mostly food) to Belgium.

The Commission for Relief in Belgium was initially started by Herbert Hoover and was largely funded by the U.S. government. It began in 1914 Because the nation of Belgium was on the brink of starvation. In 1914 Belgium was only producing about 1/4 of it's food requirements. After Germany invaded It was no longer possible for Belgium to buy food as it had done previously. France and Great Britain couldn't cross the front lines to sell food and Germany refused to sell.

The Ships initially were U.S. flagged and all food delivered was the property of The Commision for Relief in Belgium making it owned by a foreign corporation rendering it out of the reach of the German government. It only ceased to be CRB property the moment it was consumed.

With the U.S. entry into WW1 things became more complicated but by that time the world had largely come to view the CRB as an entity unto itself operating outside of standard international rules. U.S. flagged vessels were no longer allowed to carry the Cargo and U.S. workers in Belgium were forced to return home but the operation continued nearly unbroken.

The ships had mostly safely sailed throughout the war and were easily recognizable due to their CRB flags and the ten foot tall words "Belgian Relief" One ship the Harpalyce was sunk by a U-boat in 1915 creating a minor uproar from the international community. The Germans were careful to avoid the ships after the Harpalyce incident.