There is a finite amount of damage a person can take to the head and it directly correlates with brain damage. When you get a concussion, your coordination and balance are negatively affected. If you're a "fresh" fighter, meaning you haven't accumulated too much damage, then you can recover either between rounds or between fights. However, it comes to a point that fighters can no longer recover and they stay affected from the accumulated damage; that's what we call punchy.
I really hate to say this, but technique has deteriorated so much that guys who get hurt don't know how to avoid getting hurt much more. Watch basillio get creamed and hit flush in some of his welter fights then go up to fight sugar ray and take those shots well, or camacho get rocked early in his career, then against Tito, but almost never hurt at 168 despite his short stature. Even "basic" boxers in the seventies and eighties did not employ this " lets completely rely on our chins" business rios seems to do. Sure a lot of those guys got pugilistic dementia, but rios is almost guaranteed a very unfortunate later life. These guys use it to the point of damaging it beyond the power of the body to recuperate.