"In other words, instead of doing assistance exercises for 8-12 reps, why doesn't it make more sense to do them for, say 5 reps? "
Assistance work (for example, in my gym assistance work is push-ups, dips, rows, curls, fat man rows, swings, single leg work, etc.) is done for 3 reasons (as stated in the book):
Provide balance to the body.
Help you get stronger in the main lifts (and overall).
Assistance work are generally movements that don't lend itself to lower reps, lighter weight. Then you have the whole issue of recovery, joint health. As I've said many, many times - not everything can be equal. Not everything is a squat or press. Some things are done for higher reps to promote the things above and not fuck up recovery.
But as I've said before - if people want to max out for 3RM on curls or rows or whatever the fuck article makes them change their mind this week - go ahead. I don't fight this shit anymore because I can't win or care to win.