Why do basic brick boxes tend not to include many gray bricks?
Psychology

This eHow article explains quite well why people (children in particular) are attracted to bright colours. Whilst it doesn't directly relate to LEGO, or toys in general, it does make quite a few good points about the effect colours have on our choices:


  "According to a 2005 research article published by the BBC, children
  can see and even appreciate colors as early as four months old. At
  that age, however, their eyes may not yet be sufficiently developed to
  recognize various shades of color or differentiate dull colors like
  grey and brown."


This doesn't really apply to the sets you've shown which are marketed towards children aged four and over, although it may apply to Duplo sets which are made for younger children.


  "a preference for bright colors may be a result of evolutionary
  developments. Many naturally occurring foods---like tomatoes, apples,
  oranges, bananas and berries---are bright, bold colors when they are
  ripe and ready for consumption. By contrast, almost all fruits turn a
  brown, black or gray color as they pass their readiness for
  consumption and begin to rot."


Here we have another reference to an ingrained biological behaviour, It's probably insignificant considering how the majority of the bricks and the packaging are brightly coloured.


  "adults typically associate bright colors with fun, happy times. For
  this reason, many parents bombard children with brightly colored
  gifts, paint children's rooms bright colors, and deliver marketing and
  entertainment messages to children through the use of brightly colored
  cartoons."


I can't claim to know a thing about psychology but from my experience building models as a child, I know that the focus has always been on building something you can play with. A few gray pieces here and there wouldn't stop me from building a car with psychedelic mixture of colours

Building

Supposing you're a child and you ready to build something from the real world that's both gray and exciting. Well, there are a few animals like elephants, rhinos and wolves you could build but other then that there aren't that many realistic things you could build apart from castle walls, boulders etc... (You are welcome to challenge me on that point)

Manufacturing

Keeping the amount of colours to a minimum, makes the manufacturing process cheaper. Fewer part bins need to be sourced and delivered to the packaging area (although a large part of this process is automated).