I saw this in the news yesterday. What a crock! Hopefully the stores don't cave to the ambulance chasers. 4x4 nominal, 3 1/2 x 3 1/2 finished. Been that way forever.
Not actually forever. Maybe as far as you can remember.
Sizing of lumber comes from rough cut. Either by 6' diameter or similar saw or large mill band saws.
It wasn't until after the turn of the century (1900) that planing boards really became a thing, and dimensional lumber was born.
With the infusion of steam power, belts and machines, things like tongue and groove flooring came along.
Everything was built with rough cut lumber.
For siding and such it was all done by hand planing, as with hand made furniture.
After WWII when the building industry really took off things like plywood sheeting came into play for roof sheeting, cabinetry and the like.
Dimensional lumber has quite a history, and it would be more confusing to change a finished milled product size than use it's rough cut size.
Even in commercial building the steel studs are of dimensional size, not actual size.
What you also get when you buy dimensional lumber is inspected and certified lumber suitable to build with.
You would NOT get your local building inspector to sign off on rough cut lumber with no stamps on it these days!
My buddy's house I have a lot of my stuff stored in was built in 1871. 2 story farm house. You can easily tell the single floor addition was put on when indoor plumbing came in.
It's built with rough cut oak timber 8" square, no insulation at all, but heats well and stays cool if you don't open the windows in the summer. I'm sure it will be standing for a very long time. The garage is built more like an old time barn structure with lap, dovetail and pegged joints. The roof rafters are all round and lack milling of any kind. I'm sure the house was pegged and dovetailed. It hasn't settled a bit.