None of the above is true, or at least isn't the full answer for why today a "2x4" is missing an entire half an inch all the way around. The shrinkage due to drying is around 5% (and the real math there is more complicated, as wood shrinks different amounts in different directions relative to the grain), which would only account for 1/10" of difference in the thickness of a 2x4. With some species of pine it's as low as 2%.

No, the lumber industry has consistently shaved boards in order to fit more into rail cars for transport and make more money and spend less per plank on transportation costs. Various lumber consortiums determined via internal testing that the smaller board sizes are still "sufficient" for their intended purpose vis-a-vis structural integrity of stick framed residential buildings.

that's one of the common excuses that the mills quote. It's bullshit of course, but it sounds plausible so they continue to get away with it.

Another bullshit excuse is that they're providing an additional service by milling and planing the lumber for you, and that the nominal measurement is before that process.

It's all just greed. If they could get away with selling a 2x4 that was half an inch thick, they would. At least it's all standardized now.

I don't think this is true. There was a transitional period around the 1940s where 2x4s were 1.75" x 3.75", and that wasn't because wood shrunk half as much as it does today.

There is no sensible reason to sell something as a dimension before it's ready to sell, a 2"x4" should be so when sold, not when curing, it takes nothing to cut oversize to accomodate for shrinkage, or to cure and cut later at the right dimension

Modern lumber is planed, so some of that difference is because of losses from that. If you open up the walls of a house built 100+ years ago, you see these thick rough wall studs that never went through a planer. Even with shrinking, it's close to being actual 2" x 4".