Removing food removes a source of great impact, especially during war. It's a strategic resource.
If buy orders are few, then there is too much food. Atamara is pretty lively and I can do multiple trades daily if I wanted to, though this may be war driven.
Adding a resource would also have an impact, especially during war. Food is every bit as justifiable as "materials" (or metal/wood/stone).
Also, on Dwi, buy offers aren't the problem. It's sell offers. There are a whole lot of lords who just couldn't be arsed to put their excess food on the markets, and as such ridiculous amounts of food are wasted to rot, just because a whole lot of people couldn't be arsed and didn't care for extra profits.
And while I get that it was intended that some people might want to hold back food to hurt others, all of the people who hold back food just because they can't be arsed to give a damn are being a real drain on the trader game.