I realize you're saying it isn't worth caring about, but they don't even really get a small advantage, because if they were forced to make the finished product 2" x 4", they'd just have to start with a 2.5" x 4.5" board, and end up with the same amount of "extra." Yes, you could argue that you'd get less extra from each tree, but I promise you that they'd just cut down more trees, since you aren't typically going to buy significantly fewer 2x4s, or any wooden boards of any size really, for your project just because they are a little bigger. Most 2x4s are used with space between them anyway and measured center to center, so you'd just end up with slightly less space. The only thing you'd really accomplish with changing the measurements is cutting down more trees and slightly driving up the price. Buildings wouldn't get built any better or faster because of it.