Because it's completely reasonable to believe that on the edge of the most major war in History, no great care would be taken to insure that the most significant ship in the Sumter expedition would arrive where it was expected so that it could lead the mission.
You plead "colossal stupidity" as your defense? Lincoln was not a stupid man. He was in fact a very clever man, and he was given to underhanded tricks, like having the railroads bring in massive numbers of astro-turf supporters with counterfeit tickets to shout down other candidates at the nominating convention in Chicago.
And that's a lie, and you well know it.
You wish it were.
Lincoln's orders were not to attack anybody: no first use of force, so long as they were not attacked.
The orders simply say to use force if they are impeded or prevented from landing supplies. Being attacked is not required for them to use force. Just parking boats in the channel is sufficient cause for the fleet to use force.
Total nonsense, cover of darkness & calm seas would allow boats to deliver supplies to Anderson unmolested.

So they could batter the fort to pieces, but they couldn't hit any of the boats or ships that would be trying to reach it? Boats which would have been closer to them than Sumter actually was?
And there was no moon landing, Area 51 is full aliens and 9/11 was an inside job. You know, for some people the facts just won't satisfy, got to be something more...
And so you finally have to grasp at the straw of "You are a conspiracy nut because you believe massed numbers of cannon batteries would sink ships!"
I'll tell you what is a kooky conspiracy theory. The idea that people would have been able to run that gauntlet of cannon fire without getting sunk.
And exactly what was Lincoln's military experience in April, 1861?
That's right, in 1832 during the Black Hawk war Lincoln had been voted Captain of his militia unit.
He fought no battles and lead no difficult missions.
He certainly had no Navy experience -- zero, zip, nada Navy -- much less the sort of combined arms operation contemplated by Captain Fox.
Furthermore, Lincoln then was under the most intense pressure anyone can imagine and stress & fatigue never make us smarter, always make us more prone to errors, even the simplest of them.
That's what Occam's razor says, and I see no reason to think otherwise.
DiogenesLamp: "Being attacked is not required for them to use force.
Just parking boats in the channel is sufficient cause for the fleet to use force. "
But "parking" boats by itself would in no way seriously impede or prevent Lincoln's resupply mission, only actual force could stop them.
And your suggestion that Confederates may even have tried to block Union ships with row-boats is absurd and I'm sure you know it.
The fact remains that Lincoln's resupply mission had a good chance of success given calm seas and cover of darkness, or even fog.
But rightly or wrongly, Major Anderson decided otherwise.
DiogenesLamp: "So they could batter the fort to pieces, but they couldn't hit any of the boats or ships that would be trying to reach it?
Boats which would have been closer to them than Sumter actually was? "
Boats moving under cover of darkness, or even fog.
Good chance of success.
DiogenesLamp: "I'll tell you what is a kooky conspiracy theory.
The idea that people would have been able to run that gauntlet of cannon fire without getting sunk."
I'll tell you what you can do, you can prove it to yourself, pal.
Go look up and find America's best sniper, a Chris Kyle class shooter who can hit a small target using a scope from a mile away.
Now sit down and buy him a beer so you can ask him the most important question: can the world's best sniper hit a moving target that he can't see in the dark?
If he tells you "yes", then buy him another beer, and another, until he confesses the real truth of it.