That's only true for hardwood (which is often sold rough). Softwood used to be cut to 2x4, but with better milling technology, it's now cut to the smallest size that still ends up at 3.5" x 1.5" after planing.
Interestingly, in Canada, our whole forest industry is metric. The only part that is imperial is the labels (We still turn out 2x4s but they are defined in millimeters). Construction is still all imperial here though.