Why can boxed brownie mixes use so little "wet" ingredients? Be it Betty Crocker, Ghirardelli, or Pillsbury, I only ever have to use (on average) 2-3 eggs, less than a cup of oil, and a bit of water.

Somehow, these people have been able to make it so that these mixes result nearly perfect without so much quantity of "wet" ingredients.

My mind boggles as to how they've achieved this due to the fact that the majority of "from scratch" fudgy brownie recipes I've come across usually call for a lot more with respect to "wet" ingredients.

I've been trying for a while to replicate these fudgy brownies with around the same quantity supplied in the mixes. Since I don't have the exact measurements of each dry ingredient, I had to play around.

My attempts have failed and I don't understand why.

I shouldn't have to use more ingredients for something others have been able to get with less.

Why can they do it?
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.