You're right, the only dominant 7th chord from the 1,4,5 progression derived from the C major scale is G7. The only reason I can think of is that 1,4,5 progressions are usually blues or rock, which is based on the pentatonic. Most of the notes from the basic triads are there, and the other notes just add flavor.

Actually, now that I think about it - if you look at the notes for those chords, you'll see that if you combine the C major and minor pentatonic scales all the notes of the 1,4,5 seventh chord progression are included therein.

Waaala!