They come from altering the 5th of a dom7 chord, for voice-leading purposes within a normal major or minor key.

By stacking thirds you still get basic dominant chords on the fourth and fifth degree in melodic minor, the altered fifths would then be elevenths or thirteenths. Of course this is merely a theoretical approach to finding those structures as stacked thirds. The voice leading aspect would in a way justify the existence of a minor chord with augmented fifth anyway. Except that the ear tends to hear b6 rather than #5. If you listen to the sound of that voice leading: C-Eb-G-B,C-Eb-G#(Ab)-B,C-Eb-A-B over a low C basically the Ab IS a G#, an augmented fifth that leads to A, like it would be in major. Maj 7 #5 is not a major triad with added minor sixth as well. I mean, some inversions are interchangeable, as in m6 and m7b5... However, the G# DOES sound as Ab when singing it. It's hard to hear it as augmented fifth.