There CAN be many reasons, & some of the statements above are valid, but from what I KNOW to be true (my two brothers are contractors), I would like to add a couple things:
1. - Unskilled labor.
When housing bubbles start happening like this one did in the last decade or so, you see all sorts of 'get on the bandwagon' behavior (pyramid scheme psychology, basically).
You know how all of a sudden almost everyone you know was investing in real estate, hoping to make a killing?
Well, the same thing happens in the SUPPLY side of the equation - every idiot who's ever swung a hammer or fixed his own sink, suddenly thinks he can make a living building houses... and in a bubble economy, he's right!
There was so much demand for new houses, that maybe 1/3 of the guys building your house had been in the business more than 1-5 years. LOTS of newbies doing substandard work.
In other words, it's not all the materials, although, obviously, some of it is, and for the same reason - too much demand, too quickly, and the China trade.
(Don't even get me started on the half-*** skills of most of the Illegals streaming into this country for these kinds of jobs. And if you think I'm being racist or inaccurate, do you see a whole lot of quality workmanship in home building in Mexico and other Central American houses in the last few decades? Hmmm... didn't think so).
2. -New wood.
A lot of your cracking problems, etc., are because all the environmentalists have damn near shut down all logging in this country. They certainly have killed off any logging of old-growth trees, which are the BEST wood, with good thin solid grain.
If you doubt this, do a bit of searching for 'reclaimed lumber'. It actually costs MORE to buy old wood that has been ripped out of barns & old houses and buildings, than the new stuff!
So we get wood from OTHER PEOPLE'S FORESTS in other parts of the world (with the extra environmental destruction of all that pollution from ships and trucks getting the stuff here!!!), and again, the demand is so high, that the trade would basically take whatever it could get.
A lot of the new stuff is green and twisted, so months after building, it warps and cracks whatever is attached to it.