BECUASE THE WAR WAS F@#KING OVER
Except it wasn't.
The Founders hadn't surrendered at that point. The Dominion forces hadn't stopped hostilities. The JemHadar had no orders to stand down.
The Cardassians (who had switched sides) were on the verge of being attack by the Dominion.
Without the bargining chip of the cure, Starfleet would have engaged in a battle whose end result is completely unknown.
So how was the war (in any way) "over."
Becuase the Founder pretty much admitted they weren't going to win the battle she just decided to go out ion the blaze of glory becuase she was going to die anyway and figured she would just spite the Federation and it's allies by giving them a bitter victory.
Now she may have considered surrender if she wasn't dying, hell we know the Dominion would likely cave into whatever the Federation wanted to save a Founder, so all they would really need to do is capture her which they already had at that point.
And I still can't believe people are still defending a plan that only worked becuase Dukat went nuts. If he was still in charge the quadrant would have been screwed thanks to Section 31. or hell even Damar aka the guy who once convinced Weyoun to try to kill a Founder (Yes the Dominion thinks Odo counts).