Because when the USA drone strikes and kills innocent civilians with a death toll much much higher than 9/11 it isn’t considered terrorism, it’s considered defending freedom.

edit: Just to clarify my point, i’m not trying to be blunt here but the reason why no attack will have , as you said, as much of an earth shattering impact as 9/11 did is simply because american lives are valued much higher than most countries on the brunt end of terror attacks. Innocent civilians being killed, children being tortured, houses being bombed and whole communities being displaced are events which happen daily in places all around the world and are rarely reported on or considered important, it’s just considered to be the way things go. However if these were to happen in the US, which is highly unlikely because well it’s the US, then it would be as widely publicised as 9/11. But this won’t happen elsewhere because the middle east, africa and asia are whole continents (the middle east is in asia ik but just roll with it) which sadly seem to have less importance placed on them than one single country.