Simply not true.
Rock, Evans, Hemsworth, Bale (for Batman), Cavill, Manganiello, Momoa, Affleck, Hughely jacked man, Diesel, Statham, Crews, Adkins, Craig, Pratt, etc.
All action movie stars get buff. It's just that stars nowdays don't necessarily star solely in action flicks so weight changes are more prominent then in the old days of "just" JCVD, Sly, and Arnie. IMO it's much more prominent today bcs one could argue that Chuck Norris, Seagal, Banderas, Kurt Russel, Tom Cruise, Bruce Lee, Bruce Willis, Clint Eastwood and Mel Gibson e.g. were not that buff in the first place (while many of them are now becoming buff for roles).
Without Steroids they all have limits on how big they can get naturally.... The rock is an exception because he's a genetic Samaon blood freak. Most the other guys are either former pretty boy jocks or manlet juicers like Wahlberg.
Well, none of us can know for sure, but it's highly unlikely he doesn't juice. Yeah, he eats and trains like a maniac, but juice is most probably still there.
In general massive guns have taken a backseat to abs
Traps and chest, IMO. Both are easier for accomplish for a shorter period of time, and look good with a shirt on. Evans and Hemsworth being prime examples.