>This might have something to do with the emasculation of the male heroes?
Even female leads suffer from this (plot-wise, males getting emasculated is a whole different park game), my shallow observation is that writing has most of the blame. For example many action-oriented movies back then depended on the main character's personality to know in which direction the plot was going to move, aka his actions decided his fate, but nowadays plots are written to make external events the deciding factor (which is fine) but very usually with a combination of "morally/logically sound consequences", by this i mean that if something happens the characters will act accordingly to what usually happens and with "logical" A-to-B paths.
This leads to plots being similar to each other if the context is similar and without the main character being that much of a factor with the series of events, you can put Bourne on a nu-Bond movie and nothing that different will happen at all, this is a strong contrast with back in the days were main characters were either written completely (motives, pastimes, attire, quirks, traits, moral code) or you hired some actor with an already defined on-screen persona to save some time (Bronson will investigate/plan his revenge, Norris will moralfag/spare the remorseful, Van Damme will get cocky/train hard to redeem himself, Stallone will pull something out of his ass to win, Schwarzenegger will work hard from the beginning, etc etc in case of western action stars). That leads to the counter part, depending on the area audiences could either hate the entire thing because the MC is after all the one who moves the plot and/or the plot was water thin because it was merely an excuse for an actor to flex his personality, AND because audiences are lazy sometimes and don't want to quickly discern what's the moral code and motives of the protagonist to make sense of his actions especially if he's a relatively unknown actor.
This is a reason why old movies usually have fan theories of "what if x happened/had decided otherwise" because they depend on the character's actions based on their flawed-or-not knowledge (What if Seagal wasn't an idiot, what if Lee Marvin was more laid-back), nowadays because protagonists are just victims of circumstance nobody flies their imagination with a "what if the towers didn't collapse", because the entire point is them falling off and the movie would've not moved if otherwise. And because of this you can risk upsetting your actors because replacing such a character is easy, writing him is easy enough, audiences can easily tolerate him because he's not that decisive/has time to flex his personality... thus why spend extra time defining some kind of attire for him and his friends?
I can pull out a good reason to do that, to make him more memorable and easily recognizable, it has worked well in other movies: Refn's Drive character is well-known for his scorpion jacket but nobody remembers anything from his personality other than being very sheltered for no reason to the point of mental illness, but that is the actor's demeanor after all, no script involvement other than not giving him lines. But you said it well enough
>Mainstream cinema is getting more and more casual and somewhat lazy
It's as simple as that really, predictable lines because of trying to write it logically from a normal bystander perspective when IRL things are not that predictable or logical, Princip candidly went to eat lunch after failing to perpetrate an assassination on a high-level royal member a couple blocks away (or so the story says), action movies need to either pack a mean punch in the action department if they want to be predictable (eastern approach) or give the sense of surprise via character's mentality and/or sense of "adventure" to keep us on the edge (western approach) if you erase both then you are left with an almost documentary-style series of events but without the informative narration, the cool trivia or the act of knowing what the hell is happening.
Lazy writing due to everyone getting complacent or maybe to concentrate on the propaganda/indoctrination angle
Thrillers, needing a really tight script, are one of the genres that suffer from your national guild of writers being a bunch of party hogs who decide who's going to get which job depending on how much blow they snort with the pals, which is okay if it wasn't for the fact an outsider will get pelted the moment someone with power wants to buy him his work.