Because if you don't know what you're doing you could blow up yourself or "make you generally unpopular with the remainder of your platoon".
Also it is possible that not all of them will detonate. You can instead launch one or more of them to parts unknown. Unexploded ordnance scattered on ranges is highly frowned upon.
Aslo, I'm assuming from the story that they did not have enough detonators. You can reuse the clackers and most of the wire.
That was actually rhetorical on my part. My whole squad actually got the why explained to us in very small words by our SGM back in the day after an awesome range experience that started with my squad leader looking at me and saying "*expletive deleted*ck this, this is taking too long, grab the mines and det cord and follow me, I'm going to show you how we actually use these things!" and ended with an, as they say, "Earth-Shattering Kaboom".
Your points were all addressed in the subsequent remedial training.