Why do we use Dominant 7th alt term. Because the dominant 7th alt is basically the tritone sub but with a natural and flat nine and flat five with the flat 5 in bass. I mean i guess g7alt is better than reading Db7(nat.9,b9,b5)/G
You are right in that a ♭II7 tritone sub chord with a ♭V in the bass sounds essentially like a V7alt chord but your analysis of “Db7(nat.9,b9,b5)/G” being a G7alt is not quite right. In the keys you referenced a G7alt contains ♭9, #9, ♭5 and ♭13. This is basically the same as a D♭13#11 Which has 9, #11, 13. They have the same notes but they have different roots.
The root largely defines the chord and the way it functions in a chord progression so it’s better to spell a chord from the root when possible. That’s why we use the term “alt”. I’m sure I’m not alone in saying that I would rather see G7alt over a D♭13#11/G on a chart. As a musician, bass player and improviser I even approach the two differently when creating walking lines or improvising. Hope this answers your question.
We can use V7(b5b9)or its triton substitution when ever we want, just the way we like it.
There’s no rule, no law or principle why we should choose the one or the other.
Sometimes it is comfortable to progress chromatically down steps walking through the circle of fifths, especially the bass line.