As a Navy Corpsman working in the ER and ICU I saw plenty of head injuries. Sailors and Marines are often injured in training. And some live pretty recklessly in their free time. Combat helmets, motorcycle helmets, you name it -- these helped minimized skull fractures and open head injuries, but did little or nothing to prevent concussions.
And nobody knows why some folks experience concussions and subsequent consequences, including personality changes, while other folks don't. Probably the way brains rewire themselves to compensate.
A somewhat comparable situation is boxing headgear. There's a misconception that padded headgear protects boxers from concussion. It doesn't. Not even close. The most devastating and frightening knockout I've personally witnessed was in the 1970s during an amateur bout between two top level amateurs: Ronnie Shields from Texas (now a successful trainer) and the Arkansas state Golden Gloves champ. The fellow from Arkansas wore full sized padded headgear. Ronnie knocked him out cold with one shot. The poor guy didn't flinch or move for awhile.
Boxing headgear is to protect boxers from cuts and bruises. That's all. It's used during training. Occasionally amateur bouts would require them but some competitions have eliminated that requirement because headgear doesn't protect boxers from concussion, and can limit their peripheral vision, making them more vulnerable to being hit. Some amateurs will still wear headgear during multi-fight tournaments to reduce the risk of cuts and bruises in the early bouts. Then they'll omit the headgear during the finals.
Boxer David Reid was America's only gold medalist in the 1996 Olympics and was expected to have a stellar career. But he suffered a permanent eye injury (injured eyelid muscle and nerve) during the 1995 Pan-Am games, and was aggravated during the Olympics, despite the headgear and precautions. The eye continued to swell in every fight in which he took a punch, which wasn't often due to his excellent defense. Knowing his career would be shortened by the injury they fast tracked him into a world title. He had a ton of potential but was in over his head against the far more experienced Felix Trinidad.
Anyway, pardon the digression. As a former amatuer boxer in a town once the home of some great amateurs and pros, I've seen the effects firsthand of concussions, many times. Many of my boxing acquaintances from the 1970s-'80s suffer permanent brain damage from concussions, despite headgear in the amateurs, heavily padded gloves, etc. Some have struggled with depression, substance abuse and erratic behaviors that tested the limits of their loved ones. Fortunately most of the folks I knew from that era are doing well now.
Remarkably and mysteriously, others do not show any ill effects. Including me (as far as I know! -- but I retired early from the amateurs in my early 20s). George Foreman took at least as many blows to the noggin as his contemporaries Joe Frazier, Muhammad Ali, Ken Norton and others, but as of very recently Foreman showed no evidence of concussion related effects -- slurred speech, etc. Just luck of the genetic draw.