Back when football players were 5’10”-6’2”and 180-220 pounds, and not propelling themselves into each other quite as fast, the impact of the collisions was less.
One thing you need to keep in mind here, the people being hit are also bigger than they used to be. It’s all relative. If there weren’t more injuries back then, why would there be more now, sizes relatively comparative?
And the protective equipment was much inferior to what is used today.
The game is probably more dangerous now, but all the attention to possible concussions and the required protocols today significantly increased the number of diagnoses. Many diseases and conditions become more prominent once they receive far more attention and diagnostic emphasis.
A lot of players played through minor concussions in the past, which is readily admitted by those who have been around the game a few decades.