The main issue with the pre-war food is that it's been 250+ years. Sure, you might find a cache or two, but overall, it will have been scavenged already.
The honest truth is, food and water sources for anyone in the capital wastes was never seen as important to the writers of the story, So it was cut. Well, it was cut if it was ever written at all in the first place.
The story of Fallout 3 is very linear. Which means that it can be tightened up and polished, and it was. But if you go even a little bit off the rails, it starts showing cracks that are immersion breaking.
New Vegas didn't have that fully polished main story. Instead, it had a polished game world. One that felt alive and vibrant.
It's the reason why people have x amount of time playing Fallout 3, and three or four times that amount playing New Vegas.