There are many reasons why economists ignore animal suffering when analyzing agriculture. One is likely simple speciesism. There's a deeper set of reasons that I think make economists ignore animal suffering, though, and they show how far modern economics has strayed from its roots in many ways. The reasons start with this: a refusal to make a "moral" judgment.
The basic premise of modern welfare economics is that there is no way to compare how well-off two people are (what's called an interpersonal comparison of utility). This belief stems from mid-20th century economist Lionel Robbins. Of course, the belief is absurd on its face: the idea that there is no way to compare the happiness of a poor Nigerian family with an American billionaire is ridiculous. Economists have leaped from the idea that there is no way to prove or know that the billionaire is better off to the idea that there is no way to have high confidence.
In practice, few economists believe what is taught as a premise in Econ 101. Professor Blattman and just about every economist I've worked with at IPA is in that field precisely because of a belief that people in poor countries are worse off than most people in wealthy countries. Just about every measure of overall economic well-being, like GDP or the poverty rate, lumps an imperfect measure of people's well-being together and assumes that well-being is in fact comparable.
The residue of the idea that you can't compare well-being, though, sticks around, and the complete exclusion of animals from economic calculations is its most radical implication. From the perspective of an economist, a dog or a chimpanzee is no different from a chair. This flies in the face of scientific consensus. The scientific consensus is hardly revolutionary: from Greco-Roman to Judeo-Christian to Buddhist to Hindu thought, the view that animals deserve moral consideration is ancient. The scientists merely confirmed what we all could have presumed.
Interestingly, the reasons to care about animals in economics extend even further: many animals have the characteristics of an economic agent. There's an entire field of animal welfare science that, while it is still rife with bias and support for hurting animals, takes as its basic assumption that animals express preferences, an assumption that economists know as the weak axiom of revealed preference in the human case. Neurological studies of what goes on in animals' brains when they make these choices suggest that this is accurate: where we have measured it, animals' reward and decision-making systems parallel humans' brains when making decisions.
In light of these findings, it is indefensible to classify animals in the same category as furniture in economic analysis. If we do that, then the costs of a program like Bill Gates' stack up immediately. Chickens are generally killed as babies at a fraction of their age. In poor countries like those where he will give these chickens, animals are often slowly hacked to pieces when it comes time for slaughter. Before their death, as anyone who's walked the streets in a developing country will know, birds are confined in wire cages little better than American battery cages (something Bill Gates himself is working to combat).
All this suffering is not worth $5 per bird, and that's the optimistic estimate of a chicken "donation" program's effectiveness.
We can quibble about the human costs and benefits of a program like Gates', but the animal cost is glaring.
Professor Blattman writes of some imaginary Gates advisor, "They sold you on the benefits, and didn't tell you how much it all costs." When it comes to animals, the same could be true for anyone who's studied economics and agriculture. When it comes to animal agriculture, economists have been sold on the human benefits but neglect the animal costs.