Is there any specific reason why female voice actors act for male roles in anime? I've always been wondering: why do female voice actors act for male characters in anime?

For example: Mayumi Tanaka for Luffy, Takeuchi Junko for Naruto, even Goku's voice actor (Nozawa Masako), etc.

When there are such good male voice actors (for example the ones for Zoro, Sanji, etc.), why do the main leads get female actors?

Is there any specific reason or is it just the way it is?
Your question is somehow related to this question. 


  Sometimes, an animated character is better suited for being voiced by
  a voice actor of the opposite sex. Maybe a higher voice is needed for
  a male, or a lower voice for a female.
  
  A common variant for this is for young boys, usually 12 and under, to
  be voiced by an adult woman. This is because real little boys' voices
  deepen when they go through puberty. Not to mention that it's easier
  to find experienced actresses than experienced prepubescent male
  actors. The producers don't have to deal with child labor laws that
  limit the amount of time a child can spend in a studio, either. Women
  are often able to maintain the role longer, as well, since their
  voices don't change like a growing boy's.
  
  There are exceptions, of course — sometimes an adult man or an actual
  child will voice a little boy. In films, this is the rule rather than
  the exception, as voice recording for a film generally takes much less
  time than for a TV series. 
  
  And on a related note, sometimes, for comedy purposes, a deep-voiced
  woman will be played by a man. (source)
  
  Also, female seiyuu voice acting as a male character is much more
  common than male seiyuu voice acting as a female character probably
  because it is easier for women to lower their voice's pitch than a man
  to make his voice's pitch higher like that of a woman.


As to why female dubbers mostly get the main role, aside from the fact that female voices' are more flexible in acting than those of males', popularity also is a major factor. There are many female dubbers that are also in their singing career so it kinda boost up both the anime and the dubber's popularity by having that person get a main role. But I think the flexibility of females' voices are the main reason.