Why is a 7 chord not built with a major seven interval?

This is a great music theory question, and one that creates a lot of confusion among guitar players and musicians in general. The confusion lies in the fact that there are two types of seven chords built on a major triad. A major seven and a dominant seven. A major seven includes the interval which is a half step below the root. This occurs on a 1 chord and a 4 chord. A dominant seven includes the interval which is a whole step below the root. This occurs on the 5 chord. Major seven chords are always referred to as "major sevens." But, and this is where it gets weird, dominant seven chords are simply called "seven." I mention this in Fretboard Theory Chapter 10 on page 130. See the headline "Dominant Seven (V)."