Because the South chose to start a war.
The point is intent to harm, and lucky accidents notwithstanding, if you strike at someone with intent, it makes no difference if you connect on them or not, you still initiated the violence.
So when the South fired on the Start of the West and the Rhoda Shannon their attempt was to harm. So the only thing that prevented the war from starting earlier is that the North chose not to accept the South's acts of war.
As I said up thread. Lincoln swung and missed. They swung after Lincoln and connected.
More like three strikes and the South was out. Four years, massive devastation, and hundreds of thousand of lives later. Davis should have stayed out of the game I guess.
The South had no ability to make that choice. Lincoln alone possessed the ability to decide whether or not there would be a war, and Lincoln decided that there would be one.
Again, everyone told him that sending those ships would trigger a war, and he chose to do it.
The firing on Sumter was a consequence of those ships arriving. The confederates intentions were to simply give Anderson as much time as he needed to evacuate the fort, and Anderson was going to do it.
Then the initial ships of the fleet arrived, and turned everything into a combat situation.
So when the South fired on the Start of the West and the Rhoda Shannon their attempt was to harm.
Star of the West was a militarily belligerent ship. It was secretly carrying troops and munitions, and so represented a valid military target.
In other words, the Star of the West was the aggressor.
The Rhoda Shannon was just a mistake caused by everyone being paranoid.
So the only thing that prevented the war from starting earlier is that the North chose not to accept the South's acts of war.
Let's keep the timeline in order, shall we?
Burning the cannon carriages at Fort Moultrie and grabbing a ship to carry his forces into the unfinished Fort Sumter in the full knowledge that this was completely against the wishes of the new government was the first act of aggression in the war.
Do not forget that Anderson's men discussed firing cannons into the city of Charleston, which is also a likely reason why people would simply not put up with them being there.
Northern newspapers had already called for the guns of the fort to be turned upon them and fired.
Nobody wants to live with that kind of threat hanging over their heads.