Why do you think 15 minutes prior to 15 Minutes prior exists to such a level that virtually the whole military has accepted it as standard?

Because, unfortunately, some dumbass screwed it up for everyone else

There was a colonial regular that was late to formation and General Washington lost is mind.

So it's been that way ever since.

MSG (Join to see) has it exactly right. Once a sergeant or officer gets their ass chewed for one of their folks being late, then the 15 minuets prior rule goes into effect. Of course, it rolls downhill, so each successive layer of command bumps the show time up to make sure they are covered. Consequently, the Soldiers end up showing up 6 hours before the C-130s arrive for the airdrop mission. They do some training for the jump, but mostly spend a lot of time sitting around. But by God they are all ready to get on the aircraft when their squad leaders tell them to. I always felt sort of sorry for them, but remembered that we (the aircrew) showed up 3 hours before takeoff for a briefing 2 hours before takeoff, so we could be at the aircraft 1 hour before takeoff. We started engines 30 minuets before takeoff so we could get in line and taxi to the runway looking well organized. One reason why? Wing Commanders were graded on the number of on-time takeoffs, not the number of on-time deliveries to the destination or drop zone.

We have all been a part of some BS with this. I never did mind the 15min prior. That allowed the plt Sgt to pass word before formation, that way we didn't stay longer. I did have a 1stSgt that mandated 10 min prior and nothing earlier. However, he always had a "meeting" before formation and would sometimes have us all wait an extra 30 min for him. ***note that in the Marines, the company/ battery gunny held formations. Not the 1stSgt.
I never did understand drawing weapons at zero dark thirty to depart hours later.

Because some idiot along the way thought they would fix things by telling people when to arrive by pre-adding 15 or even 30 minutes prior to the time they pass along. Just be transparent on what time events start while also reinforcing that 15 minutes prior is expected. Your message gets across, mission is met and you don't have people showing up 2 hours prior because of some well intentioned moron who wants to earn an achievement medal.

What you are describing is the marine rank based meme on the original post. I was introduced to it as the "Squad Leader Effect". Everyone in the chain backs it up 15-30 minutes at each level, so you are late before you ever start. it is firmly entrenched because no one wants to proof the engagement area around the CSM's desk by having anyone in their formation being late.

SPC (Join to see) The civilian world expects you to be on time for meetings or events. I had one boss that would have 0700 meetings. Show up late and it would be reflected on your performance review. I conducted many meetings during my civilian career and expected folks to show up on time as I would start my meetings on time. I prepared for every meeting that I conducted by having dry runs to ensure that everything went smooth and computer equipment worked. I always arrived to meetings early and was prepared to fully participate. Before every flight, we had Flight Readiness Reviews that would last from 6 to 12 hours. You better not walk in late. My presentation portion would usually not last over 15 to 30 minutes and I was prepared to answer questions. Once you had finished your presentation, you could leave the meeting.

I had one boss that would hold weekly staff meetings that would last two hours. He would talk business for ten minutes and then talk about his kids or something else. I provided him with very detailed weekly activity reports, so I would find ways to miss his meetings. I would schedule my meetings at the same time as his so I had a reason to not attend his meetings. During my career, I usually worked over fifty hours a week and did not appreciate someone making me work longer hours.

Over a 33 year career, I chaired or attended many meetings. Planning to be early to a meeting or event will ensure that you are on time, every time.

Because if you tell a soldier to be somewhere at a specified time, people will show up 1 minute prior and then it takes 15 minutes to form up.

Even coming up on 20 years retired I try to be 15 minutes early to all appointments. Throws non-military people for a loop. LOL!

Had a SSG at Bad Tolz NCO Academy show up up 1 minute after formation formed and after 30 days of school he failed,failure to follow instructions (FFI minus 5).