For comparison, sheet goods aren't priced this way. A 3/4" sheet of plywood really is 3/4", because the plywood was manufactured that way and as a manufactured product there is no seasoning.

You're not being ripped off because a 2x4 is actually smaller. A sawmill had a tree, and they cut off a hunk of that tree, and they charge by how much wood they cut off. The dimensions of that piece of wood changed after they cut it (which in and of itself is a service they provide - it would be a hassle if you had to buy green lumber and dry it yourself).

It's interesting that lumber kind of looks like shrinkflation (what the comment I was originally responding to suggested), but it's more that technical improvements have allowed producing the same finished product with less input material. But we still label the stuff by the amount of input material it used to take for historical reasons, which at a glance looks like shrinkflation.

And what's is worse is how hard it is to know the true size of everything you find in stores in the USA. The same applies to pipes of all types. But even things like a drawer slide is often not the advertised length. Saw blades thinkness is converted from 2.5mm to whatever the closest inch the salesperson felt that day.

From having spent quite a lot of time in big box stores in France and the USA. I can tell you that in the USA 3/4" (19mm) can be anything from 16mm to 24mm.

In short almost everything is built in metric and the USA likes to pretend it's not and gets confused by rounding up randomly.

3/4 plywood will measure less than 3/4.