If you just want to be lean you can achieve that with diet alone. But if you want a lot of muscle mass, then you need to exercise. If you gave an entirely sedentary person anabolic steroids, they’d probably end up with slightly more muscle mass then they otherwise would have had, but you’re not getting a gorilla physique without a lot of exercise.
I want to lie 12 hours straight under a tree and be reasonably fit, like a gorilla.
Eating healthy makes sense but excercising, by definition of that word, does not.