Why ? Well, large scale homebuilders profits rise in direct proprotion to speed of which their homes are completed. Generally speaking, homebuilders incent sub-contractors to finish on-time and penalise them when they do not. Corners are cut in an effort to maintain the all important schedule. Unfortunately, the builder cannot watch every sub-contractor at every single stage of the build.
Hiring an independant home inspector will give an extra set of eyes and make sure your interest is represented. They have expertise and will recognize deficiencies. Plus, they are very affordable and pay for themselves many times over.
I had a colleague who bought a newly contructed home just a over year ago. And, at my request , he hired a highly respected home inspector. During the roof inspection, the inspector noticed much of the roof insulation had fallen during a wind storm. He took some pictures and decided to check back a few days later to make sure the insulation was reinstalled. A few days passed when he decided to take another look. By this time, the drywall was now being installed. When he arrived, he noticed several sheets of insulation still laying on the floor. Upon closer inspection, he found that the ceilling still had missing insulation. The trouble is, that the drywall had already been installed over much of the ceiling. Of course, with the pictures in hand, he brought this issue up to the builder and it was rectified. Imagine the added costs to the energy bills had the insulation issue not come to light. Best money my friend ever spent.