Different weight goals
Goal of a bodybuilder is to have as much muscle mass as possible while maintaining as little fat mass as possible. This involves getting slowly heavier over time.
Being heavier in other sports may be detrimental. It makes calisthenics harder, so gymnasts and other calisthenic athletes typically want to keep their bodyweight lower so it's easier to do flips and stuff.
In contrast, strength sports also typically have weight classes that athletes have to compete in. It makes you more competitive to stay in a particular weight class and be as strong as possible, so strength athletes will try to maintain their body weight at a certain level while trying to make the muscle they have as strong as possible.
Though you compare a strength athlete that doesn't have weight classes (strongman) to a bodybuilder and you get a different story:
(Note: The strongmen are slightly closer to the camera, but they are certainly still way bigger).
In the image you have Brian Shaw (left) and Zydrunas Savickas (right) next to 4x Mr. Olympia Jay Cutler (center). Now, neither of these people are natural, but they all have similar goals. To get as big as possible. Natural athletes competing in the respective sports would show similar results (just smaller and a little more bodyfat). The difference is strongmen don't particularly care about their bodyfat levels, so they don't spend half the year cutting to extreme lengths. This allows them to just continuously grow unimpeded by overly restrictive diet.
Differences in training styles and focus
You are correct in that rep-ranges don't affect hypertrophy as much as previously thought (at least below 40), but you still need volume to get enough stimulus to grow muscle. A strength athlete is trying to get as strong as possible, they need to lift as heavy as possible. Lifting heavy in compound movements is far more tiring, so it's likely that they don't lift as much total volume. For example, a comparison may be a typical 5x5 squat routine at 300 lbs, you'd lift like this:
300x5 + 300x5 + 300x5 + 300x5 + 300x5 = 7,500 lbs lifted in a session.
vs a bodybuilding style training where someone of similar strength may squat 150 lbs at 15 reps for 4 sets you get
150x15 + 150x15 + 150x15 + 150x15 = 9,000 lbs lifted in a session.
On top of that, a bodybuilder is going to do weight training geared to generating more volume which will build more muscle. A strength athlete in contrast may do accessory work to fix imbalances and train technique. That kind of work doesn't necessarily build more muscle.
Genetics plays a big role in one's potential to build muscle. There's a selection bias involved with bodybuilding because people with above-average genetics are going to compete. The people with the best genetics will win more often. So the people you see on stage are going to be people with good genetic potential.
One bodybuilder that we can use to compare what's possible naturally is Greg Doucette.
In that image, he claims to be natural. I trust that he's telling the truth only because of how flagrantly open he is about his history of drug abuse.
Another show of genetic potential is Chris Bumstead at 18 years old.
According to him he wasn't even training for bodybuilding at the time that photo was taken. He already clearly had a lot of potential. The reason I believe him when he claims natural in that photo is because he's Mr. Olympia Classic Physique. You don't become the best in the world with mediocre genetics.
Yes. You're right. It is possible to take steroids for years, come off, and pass drug tests for bodybuilding shows. You're not going to be as big as you were on steroids, but you will be bigger than you would naturally. It's not everyone, but yes. People do it. To pretend otherwise would be wrong.
However, just because someone has an impressive physique does not mean they're on PEDs.