What's so important about keeping military operations secret?

They involve killing people and you gotta make sure you aren't the one being killed which is easier to do if your enemy doesn't knows your plans

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 Which means that any assumptions your plans make might be outdated or even actively countered.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.

A few things
If it's an operation you end up not committing to you don't want the infamy of having planned it.
Advanced knowledge of the strike may allow the target to reposition to avoid or oppose the strike.
Persons involved in the strike may be endangered due to the association.
You look like fucking ass clowns.
- #4 still applies even if you already looked like a "fucking ass clown" before. Fuckingassclownery is limitless!
  I would only add that depending on size it may not be possible to keep an operation secret. D-Day or Gulf War 1.0 come to mind when the world knew it was about to happen, maybe not the exact hour but we still knew. And then it's a game of obfuscation, i.e. deliberately leading enemies down garden paths so you can surprise them with your real plan. But you wouldn't want to leak your disinformation campaign in your text group either.
  
  D- day is a great example of why opsec matters so much. The Germans knew that the allies were going to invade, and if they had been prepared they very well might have rebuffed the invasion. But the secrecy worked, and operation overlord succeeded instead of being a bloody failure.
  If the target of the military raid had known when it was coming, they could have simply relocated anything actually important away from the target zone.
  A useful analogy is probably a boxer and a ring: your opponent knows that you're going to throw a punch, but you really don't want him to know exactly what punch you're going to throw when.

Next Monday, at 6:30 5 lorries will leave the main base to bring supply to the outpost, these vehicle will take the highway 13, then the road 666, One hour prior to the operation, mine-sweepers will inspect the bridge, and special forces will secure the pass to prevent an ambush.
Now the enemy knows exactly when to attack.

The name of the comm is rhetorical, you're not supposed to try and disprove it.

Attackers get to choose where and when to attack. Defenders get to dig in and prepare defenses.
The attacker don't want the defender to know where and when, because then they will just reinforce that area. Defenders don't want the attackers to know where and when they are weak (eg troop rotation)