Why is the quality of trades so poor?

On an inspection this week we found nearly every wall to a 5 bedroom detached house wasn't plumb, which to us was noticeable by eye on arriving.
There are tolerances set by the likes of the NHBC [-link to a useful doc showing the more common issues we find] but it begs the question why do we find so many fundamental issues?
The answer in our opinion can realistically be associated back to the recession of 2008 when a lot of established trades folded and never returned. 
The demand thereafter coming out of the recession relied upon apprentices with little experience who are under pressure to build quickly.

NHBC Guidelines: Chapter 1.2

The attitude that a lot of builders will give in response to rectifying issues is that they build to tolerance. This is troubling to hear considering for an issue to be out of tolerance it has to be quite severe (noticeable to an untrained eye), so why do they aim to produce houses that are only meeting this standard?