Just curious as to the reasoning, is it a spec, or is it simply 'backwards compatibility'?
The 2x4 refers to the rough-cut green wood: it shrinks during drying, then the dried wood is planed smooth, so the finished lumber is supposed to end up at 1.5"x3.5". While it doesn't really shrink that much, the mills get more usable finished 2x4's from a given tree if they cut them slightly smaller to begin with.
Here's the Wikipedia article about it.
The reason given these days is quite a bunch of BS. The real reason is the lumber mills decided some decades ago to cut green wood smaller - getting more boards out of a log. The shrinkage continues even now as you'll notice all lumber sizes are shrinking even with the "reason" that was sold to the public as to why the boards are not full measurements.
It was in the late 60's, early seventies they started shrinking more and more. The norm prior was you asked for a 1x or 2x and you received a 1x or 2x full plus a 1/16"-1/8". The 1/16" or 1/8" inch additional was given as most would want to plane the boards to the full thickness.
Same was for the width. It was always the full measurement plus an 1/8".
And yes, everyone questioned why the boards where not coming in full anymore. The reason given is what has been spread around and continues to be spread around as truth when in fact it couldn't be farther from the truth.
That's why today, at the time of this writing a 1x is barely over a 1/2".
It's shrunk from 1 1/8" to 1" to 7/8" to 3/4" to 5/8" to .61" at its current state.
The Mills took into account shrinkage when cutting way back. They took into account warpage, and planning to achieve a full 1" thick board.
The excuse they give is like saying the bottle of water you bought is only half full because it evaporated on the way to the store.
Or the extra large shirt you bought shrunk after they washed it before shipping out to the store therefore it's a medium.
It's all BS and you're getting ripped off just like the rest of us, that's why they're not the correct size these days. Sad but.. that's the truth.
You might find this interesting: History of Yard Lumber Size Standards
From the summary
Economic pressures among the regional areas of lumber production have resulted in a decrease of standard lumber sizes over the period covered by this history.
It goes into a lot of interesting detail.