Because on the civilian side it's a business, and most trainers would quickly put themselves out of business if they were that rigid.
From what I've seen, it would happen a lot if checking off boxes at the beginning were a go/no-go.
Perhaps some instructors pride themselves on being hard-core about this thing, but I've been to eleventy billion various classes and I've never seen it. Most instructors don't even do a formal emergency procedures brief.
I imagine it's different if you're an LEO or .mil instructor and guys are required to show up and if they aren't 100% prepared he tells them GTFO. I can see that.