Must be a local demand thing... I have 100+ of treated boards just hanging around, and I just bought several lengths of treated 4x4's and 4x6's for a job I'm on, no issues here.
One note on the quality / dryness / grade of lumber, it's also a demand issue, demand is so high for lumber that mills have to pump them out at a phenomenal rate to keep up, they have had to cut drying time to a bare minimum.
I live within 50 miles of 15 mills and not a single one is anywhere close to slow.
Average trip to town I'll pass or be passed by 10 logging trucks.
There is also the issue of, you get better lumber out of large trees, well we are cutting smaller and smaller trees every year to keep up with demand.
Yea they is a little bit of the money aspect of it as turning out more lumber per hour makes more money, but i really don't think that's the real driving factor.
Most common big box store lumber is #2 bordering on #3
A lot of the mom and pop stores are forced to do bidding on lumber buys and then it's a crap shoot on what they end up with.
If you want really good wood pay a little (to a lot more) and get Select Straight (SEL STR) or gold standard wood, it will be large knot and twist and bow free (or at least very minimal).
In my house build it's all Select Straight and Gold standard wood, granted I did make a heck of a deal on 4 Bunks of 2x6x16 and 2x6x20 5 years ago which went way up in cost now.
Kind of the old saying, you get what you pay for!