Because surprise is important, and if the enemy has precise intelligence on what’s going to happen they can act to make it not happen. Which means that any assumptions your plans make might be outdated or even actively countered.

To quote Sun Tzu, “All warfare is based on deception.” The lengths militaries have historically gone to in order to keep operational security or obfuscate the details of an attack is utterly absurd.

A real world example: In WW2, ahead of the allied invasion of Sicily the British launched Operation Mincemeat. They took the body of a homeless person that had recently died, gave him an entirely fictitious service record/life, and some fake letters heavily implying that the allied invasion of Sicily was a feint and the true invasion was going to be in Greece and Sardinia. Then they took the corpse onto a submarine and let it go where the tide would take it to Spain. The Spanish shared the letters with the Germans, and the Germans then reinforced… all the wrong places. Which made the Allied Invasion of Sicily easier than it potentially could have been.