Why can boxed brownie mixes use so little "wet" ingredients?
I've seen an awful lot of from-scratch brownie recipes with 2-3 eggs and a cup or less of butter/oil. I wonder if you're comparing recipes of the same size? 

Beyond that, although brownie mixes may look like just dry ingredients, if you look at ingredient labels, they often have oil as an ingredient, so you may not be adding quite all of it yourself. Sometimes they also  have emulsifiers that can help out with one of the roles that eggs play. And it's hard to judge from ingredient/nutrition labels, but they often taste very sweet to me, so I wouldn't be surprised if they have a bit higher ratio of sugar and a bit less flour/starch to need to hydrate.

Anyway, if you're trying to replicate, it's probably easier to start from an existing from-scratch recipe and tweak it than it is to copy the wet ingredients and invent the rest yourself. It may be hard to get exactly the same results as you can with a mix that contains a lot of fancy stuff (emulsifiers, different starches, maybe corn syrup...), though.