Translation: it's made from the smallest possible tree you could plausibly saw a 4x4 out of. And the pith (the center of the trunk) running through the middle guarantees that the post will split along the radial plane as it dries.
If you go to a better lumber yard, you get better lumber. Except, apparently, for 2x3's. They are universally crap. Source: built a chicken coop with 2x3's and bought from a local outfit because they deliver inexpensively. Everything else was gorgeous compared to the box home improvement store. The 2x3's were pretty much the same.
Makes sense that the larger cross-sectional lumber needs to come from a larger cross-sectional tree with few defects. The crap-wood they can turn into 2x4, 2x3....
Wood can have a fair amount of internal stress in it, and cutting it can release it in ways that cause your boards to warp, twist, or bend.
(Freely available to download for anybody who's curious)
https://lostartpress.com/collections/books/products/the-anar...
To get that beautiful wood again, we would have to plant the forests, let them grow for hundreds of years, then harvest the wood. Well, that's not going to happen, so we end up in the situation we are in of having to use a very poor substitute.