Injuries, life, and the fact that it’s not possible to continue progressing forever. I wish I could add 5 lbs to my squat workout every week forever.
Another reason is that strength is a skill and requires neural efficiency. I bounce back and forth between strength and hypertrophy phases. I lose some of my efficiency when I stop training the big three with heavy weights.
And lastly, I think what most of us consider jacked is a bit skewed by pharmaceuticals. I don’t have a problem with people who use, but the fitness industry has tricked us into thinking that those jacked models are natural and just train harder than us. The reality is that they’re on just a little bit of gear…about a half cup.