Because foreign aid doesn't build resilience, it builds in a dependence. And the primary purpose of goods based aid is to stabilize the prices of certain goods by destroying surplus. Foreign aid just looks better than piling it up and dumping kerosene on it.

It's important to understand that every field, which we can call farming capacity, will on average produce a similar amount as any similar field. However, this realized production varies year to year, and in a given region this variation tends to trend the same direction. It's also not entirely predictable. Bad weather, new pests, etc. can cause low production.

It's also important to understand that this is a marketed set of goods. Everyone wants grain, especially people who can turn 1.5 tons of grain (price 450$/metric ton) into 1 ton of chicken (price 1500 per ton). Chicken producers(general corporations, not small farmers) remain grossly profitable even at high grain prices. People in a developing economy struggle to afford market rate grain in good years.

Dumping a pile of random stuff on people kills the local market for that stuff. To a non industrialized economy, being used as the dumping ground for grain surpluses and clothing surpluses kills two pillars of the local economy, farming and fashion. So when short years come you don't have the farming capacity to cover the sudden shortfall or the economy to outbid richer nations with a production shortfall. You don't have the local economy to properly signal demand, because no one has anything to exchange of any value or rarity.

It doesn't help that most of them are export driven national economies that have had large losses of arable land to climate change, destruction from mining and drilling, or usage for cash crops. Those cash industries feed the nations in fat years, but in short years they can't afford it.

And even with local production, it's a market good. Without a local government that wants to trap local production of grain it will be brought to market, bought, and shipped to the highest bidder. If the chicken corporations can still get a good price for chicken they can still outbid the locals on grain. It's sadly often in the best interests of everyone in control of African nations farming to export all produced goods, even in time's of famine, to enrich themselves with foreign exchange.

The purpose of food aid has always been to stabilize grain prices without destroying capacity, so in short years the capacity fills the host nations needs. Africa is simply being used as a spring, crushed when times are rough so no one else feels the bumps as bad.