I think that this is because modern weaponry is easy to use, by design.  Marksmanship, to practical engagement distances with modern weapons and optics, is the very definition of a “low skill floor, low skill ceiling” activity.  Easy to learn, easy enough to master.  I’ll swear on anything you like that you can score in the top 90th percentile of the FBI’s handgun qualification exam with just two weeks of training.

On the other hand, boxing has a very high skill floor.  You can remain a novice for years.  Some men are physically or mentally incapable of ever becoming decent boxers.  Boxing also has an insanely high skill ceiling.  Success requires every element: Dedication to training, good coaching, mental resolve, and physical gifts. 

(As an aside, that old mainstay of infantry, the spear, has a relatively low skill floor and low skill ceiling.  Fighting with the spear is simple, intuitive, and easy to master.  The sword, on the other hand, is just the opposite.  It’s like boxing.  Perhaps it’s even more complicated and subtle than that.)  

Training, or play, that sharpens the mind and body — and weeds out the physically, socially, or mentally incapable before they show up at the recruitment office — would be excellent as a foundation for any country’s armed services.

Which is why professional militaries spend more time on weapon systems than CQC. In a real way we won’t bother clearing buildings, better to drop a city block with arty than lose a platoon to snipers.

Rohm the best friend of Hitler a physical culturist and homosexual who liked strapping muscular young men as “partners”.

From .. Psychol Sci 2013 Jul
The ancestral logic of politics: upper-body strength regulates men’s assertion of self-interest over economic redistribution
Michael Bang Petersen 1, Daniel Sznycer, Aaron Sell, Leda Cosmides, John Tooby

The various authors have published more since then, but that’s the foundational paper I think.

I have little experience with jiu-jutsu, but my father had me train in boxing throughout my formative years. (In fact, some of my earliest and fondest memories are of that suburban strip-mall gym, in the late 80s and early 90s.)

The thing about boxing is that it takes a hell of a lot of practice to get the basics down. Footwork, keeping your hands up, rolling with punches, understanding your range so that you can throw a jab effectively and without pawing like a noob… The average man off the street can train in boxing every day, for a year, and still be a total novice.

Secondary to technique, but simultaneously with technique, training in boxing teaches aggression, resolve, discipline, and keeps you in good physical condition. It also teaches patience; you can’t “overdo it,” and you can’t rush things, or you’ll get injured — hand, wrist, shoulder, TMJ, concussions, whatever.

In contrast, most forms of practical marksmanship are extremely easy to learn. The average man can learn how to shoot competently within a few weeks. Most people can pass the FBI’s handgun qualification — which is harder to pass than most normal police qualifications — with a week or two of daily training.

And marksmanship training is so easy, so removed from hard physical effort, that it doesn’t require or develop grit.

Excessive marksmanship training has always been quite pointless. Rucking, land nav, and other forms of physical training are more important, for the soldier. Even games, for instance, per Wellington’s famous quote that the Battle of Waterloo was won on the fields of Eton. The right games and sports develop qualities in men that make them good soldiers.