Because there is no direct evidence. It's mostly speculation with a few unsubstantiated claims about the evidence.
Iacoletti,
Given the layout of the TSBD, the fact that the motorcade route incorporated a near-hairpin-turn virtually in front of it, and the fact that the Secret Service follow-up car had just completed its turn onto Elm Street when the first (missing-everything) shot rang out (half-a-second before Zapruder resumed filming at Z-133), etc, what kind of direct evidence would you expect to find if, indeed, the assassin had fired all three shots at JFK from the sixth-floor Sniper's Nest?
Adam's and Styles' encountering him or her on the way down the stairs?
Truly's and Baker's encountering him or her on their way up the stairs?
Anything else?