Last month, in a camp set up for flood refugees outside Pakistan's southern city of Sukkur, a group of men and boys gathered around the medical tent complaining about the rising cases of stomach infections. "They give us food that's too spicy," they said.

"What do they give you?" I asked a young man.

"Korma," he said. "But they put too many spices in it. We don't like these spices."

A relief worker at the camp who overheard our conversation cursed under his breath. "They get to eat korma every day and still they complain." The implication was clear: could they afford to eat korma before this flood made them homeless? Shouldn't they be grateful?

I heard a similar refrain later in the month when I tried to explain the scale of the devastation to a businessman friend in an area that had been spared by the floods. "Did the media ever report on how these people lived before the floods? They lived just like they are living now: on the road, without running water, without toilets."

It's not lack of sympathy, but lack of imagination. Most of the 20 million people affected by the worst floods in the country's history lived in abject poverty before, but they didn't live like this. They weren't homeless, chasing charity trucks on the country's highways. They had a roof over their heads. The reason we didn't see them on our television screens was because they were busy, eking out a living from the land and fussing over their buffalo, their goat, or a few chickens. I grew up in a village where a whole family could be raised - children sent to schools, new clothes bought once a year, daughters married off - with the income that a buffalo's milk brought in.

In the initial days of the floods, many rescue workers were angry and frustrated when people refused to be rescued without their cattle. Not because these animals were like family members, but because they were their only revenue stream, their life insurance and their children's future. This was the kind of poverty where people might get to eat korma only at weddings or their landlords' funerals, but they had some control over the combination of salt and chilli powder they put in their pots. It's called dignity.

A television journalist accompanying me on a visit to the flood relief camps spent a whole day trying to capture the lives of children in these camps. In the evening he told me that he felt disappointed. "All day I searched through my lens but their faces, their eyes, don't have the kind of desperation, the suffering, you see after a disaster of such magnitude." He said this like a committed professional who wasn't able to do his job properly. Maybe it wasn't just lack of empathy, I thought. It was a kind of emotional blindness caused by watching too much distant suffering on our television screens. In an era in which unmanned drones carry out Nintendo Wii-like wars and 16-year-old boys blow themselves up in mosques and shrines, we want our disasters neatly packaged. We expect our tragedies to look spectacular and last around five minutes, before we are regaled with a human interest story with an uplifting ending. There have been those, of course: a cow did save its owner's life, and a woman gave birth on the roadside before she was shifted to a camp and given lots of baby clothes. But 500,000 women are likely to give birth in similar conditions during the next six months, and seven million people are still without shelter, three months after the floods began.

Mostly what we see on our screens are millions of people living under a charpoy, the family bed, often the only household item they managed to salvage before water took everything away. We see them huddled around a makeshift stove, boiling rice, borrowing a pinch of salt from each other. Sometimes we see them muttering that it's Allah's will. Very occasionally, they are blocking a road to protest at the government's indifference. We see young mothers who have aged rapidly, herding their children to a lone hand pump, forcing them to take a shower. They have not had much training in how to look miserable for our TV cameras.

The list of what flood victims need to rebuild their lives is astonishingly short and inexpensive: seed for the next crop, fertilisers, some form of subsidy on electricity and irrigation water, and, if you want to be really generous, some financial help to rebuild their homes. The lucky ones were handed about £170 in cash after they were forced to return to their still-flooded homes. ("Did they ever see so much cash in their lives?") Many more have just given up and gone back to their muddy farms and collapsed houses.

Pakistan's government and its friends have failed these 20 million people. Saudi Arabia and the US, two of Pakistan's oldest and closest allies (and, just to remind ourselves, the countries at least partially responsible for Pakistan's many raging battles), have together promised Pakistan $600m in flood aid. Yet only this week they have agreed on a $60bn deal for military hardware that includes, among other things, Black Hawks and Little Birds. You can't make korma with those.

During the first few weeks, the UN secretary general Ban Ki-moon called the flood a slow-moving tsunami. It might have disappeared from our TV screens, but it is still moving. What survived the flood will be destroyed by our collective lack of imagination and our shrinking attention span.

Photographer Gideon Mendel travelled with ActionAid. See his Pakistan film at guardian.co.uk/video.