So during the Last Jedi
Luke does his force projection and faces Kylo Ren. He has a brief conversation with Leia obviously before he does this.
Whilst he's fighting Kylo, Poe says something along the lines of it seems like a distraction so they can escape.
Now in my mind I think Luke knew that Kylo would want to face him. He probably didn't anticipate Kylo trying to shoot him with everything he had first (which probably would have wiped him out if he was actually there). But at no point did he say to any of the rebels. "Hey, I'm not actually here but everyone thinks I am, so I'll distract Kylo Ren and you guys escape out the back, and boom, Bob's your uncle and no one is hurt".
If Luke was creating a distraction for everyone to escape, why didn't he say so rather than relying on someone to figure it out, regardless as to whether he was actually there or not. People could clearly see and interact with him. For all he knows they could have thought all hope was lost and just decided to watch the battle hoping for the best.
I'm looking for an in-universe answer/explanation rather than cinematic reasons.