The reason for "startup" to include the first active frame is because the primary use of frame data is punishing, links and frame traps, and the idea that a 5 frame move punishes -5 five moves, combos after/is a tight block string after a +5 move is way more intuitive than the KoF style where you need something listed as 4 frames to punish -5 moves/combo after +5 moves.
The only place where this actually introduces confusion is when people are talking about moves with no "real" startup frames being listed as 1, which is an edge case, so no, nobody's going to change the convention any time soon.
It would be nice if they were listed as i5 or i1 instead or startup 5 or startup 1, Soul Calibur style, since that's more explicit about what's going on, but whatever.