There is risk of brain injury from repeated hits in rugby, the risk is not zero and part of that is sub-concussive impact. There are lawsuits taking place currently in England around the issue, but those players played in an era where there were no substitutes (even for injuries, if you lost a player you played a man down), and players just clattered into each other at a sprint in lots of parts of the game. Even once substitutes became a part of the sport, the was (and of course still is, in some places) a culture of playing through "head knocks" and other injuries.

That said, rugby really has changed a lot recently, in order to try and preserve the sport while protecting players. Revisions are still ongoing, and only time will tell how effective they are. But it's a very different attitude toward head contact and brain injury than football has, where they still routinely use their helmets to make hits and have just removed the most flagrantly unsafe things (helmet to helmet targeting).