You seem genuinely curious so ill attempt to answer. Theres two types that use pain killers. Clueless newbies and seasoned vets. Seasoned as in 15 years plus serious training. For the latter you need look at nagging injuries. Also for the latter those that train on elite levels. Personally sometimes im squeezing in 2 workouts a day 7 days a week. Pain can become a problem. Also youre definition of a brutal workout and a seasoned vets are in two completely different worlds. Thats just being honest. I see what most on the internet consider brutal and its laughable. Example, if You see what pros lift and think its not possible, all drugs, or some other reason, your training isnt what you think it is..
Just my opinion

I have over 15 years of training straight, no breaks, no injuries and yes, my workouts have been brutal.
I have posted my workouts many times before and most people don`t think I do them, but I really do.
I have done tons of volume, 10 sets of 20 squats- 1-2 minute rest, GVT training, Supersets, Dropsets, trained until you see starts and everything is spinning and the only thing you hear is your heartbeat on the verge of passing out.
I have done it all and still do. Always train hard.
No injuries, no pain. I have done Smovlov, squatted, no leg press for years now. You get the idea. The way I see it, some people are just broken. Glad I am not.