According to my sources:

The Dominant is so named because it was the 'Chanting note' in plainsong (early church music). It was NOT necessarily the 5th note of the scale, there was a period of 'the wandering dominant' until it settled down on the 5th.

When western harmony evolved, it was found to be easiest to harmonize just one of the 'greek' modes, which became known as the Major scale.

Part of the reason for this is that this scale has a 'leading note' a semitone below the tonic. The chord sequence (cadence) V to I, e.g. G to C, in the key of C has a 'finished' sound to it (aka full close), and adding the leading note to the chord V makes it V7.

Although this is technically a dischord, it is resolved by the following chord, and western ears have become used to it.

If you are playing in a 'mode' such as Aeolian or Dorian, you will use the same chords as the major, but will not finish the tune on the tonic, therefore the 'Full close' is not appropriate, and you should play, for example, G instead of G7. The Aeolian cadence has a quite different sound, as the 'leading note' is a whole tone.

It is for this reason that the 'harmonic minor' key was evolved out of the modes. In that scale, the leading note is sharpened and you can once again use the dominant 7th.

Hope this helps