Why is the 2 by 4 getting smaller and smaller? [video]
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First, info for US readers, 'lumber' is usually referred to as 'timber' in other anglophone countries such as UK, Australia and NZ.
Second, in those countries 2" x 4" is usually referred to as 4" x 2" — I don't know why the order became reversed (if someone knows then tell me). Moreover, those Imperial terms are now deprecated as most countries are metric or have gone metric in the last 40—50 or so years.
Third, the US is now essentially the last country to use Imperial units although you'd have to wonder about the UK as so many there have still clung to the Imperial system. That said, many of those in anglophone countries (in addition to) the US would still understand expressions such as I'm going to purchase some lengths of 2 by 4 or 4 by 2, or I'll take to the bastard with a bit of 4 by 2.
Forth, as most of the world is now metric, when doing internet searches it's useful to know that 4" = 101.6mm, 2" = 50.8mm (nominally 100 and 50mm respectively). The link below for Australian timber sizes shows how complex this can get, for sawn timber 4x2" is equivalent to 100x50mm which becomes 90x42mm for dressed/finished timber but not so for, say, Radiata Pine Kiln Dried Dressed which is 90x45mm. Note the dressed sizes are slightly different to current equivalent US sizes which is significant if timber is imported from the US and vice versa). https://harpertimber.com.au/industry-standard-timber-sizes/
Similar tables exist for the UK and other countries—and also one cannot assume the standards to be the same. You'll also note from the table that timber is specified by width first, thus 100x50mm is a hangover from the days of 4x2" (during metric conversion there wasn't a move to specifying height first as does the US which makes me think there's more to how this order came about and why it's different to the US than just happenstance).
Incidentally, outside the US a 'plank' is usually defined by having a width of significantly more than two times its height so 4x2", 90x45 or 90x42mm wouldn't be classed as such.
As I said, it's messy. Australia has been metric since the 1970s and unlike the UK most people now automatically think in metric units (incidentally, there's a good reason for the difference in take up). Go to a building site nowadays and start talking Imperial units and you'd get some strange looks (older hands would say 'where have you been all these years?').
Presumably, if you go to a hardware store where you are then you're only buying dressed timber. So standardized rough-cut (undressed) sizes must be larger there, that is 100x50mm would have to be at least 5mm larger, if not more.
Wherw I am in Australia the main reason for dressed timber being, say, 90x45mm is to ensure that ripsawed/rough-cut (here 100mm) after being properly dressed has no imperfections.
From my experience, ripsawing timber (cutting logs along the grain) results in a rough-cut where variations exceed 1mm perhaps even 2mm due varying grain hardness, ripsaws having fewer teeth per cut and drying shrinkage. Also, rough-cut here is likely to have more variation due to the coarser-grained timbers usually encountered here (timbers with a grain strucure similar to, say, Pinus sylvestris/Baltic pine or finer are not as common here, so one expects the allowance for waste to be higher).
Still, from what you say, it seems end consumers do better in metric countries than here—you end up with more product. It likely also makes better sense from a marketing perspective as sawmill/wholesale sizes can be kept separate from the retail market.
To me it just seems kind of obvious you can't just... lie about the size of what you're selling. Sure it's a natural product and there are variations but you can't just be wrong about the size intentionally.