And heavy drinking and drug use doesn't?
What about players boxing in the off season?
There are 2 things players do to harm themselves.
Now weigh that up against the changes the game has made to make it safer. So the game is taking steps to minimise risk
I will use Paul Gallen as an example
Lets say in 20 years he gets CTE.
He is a 300 game NRL player but you can't say it was 100% NRL that caused it.
It may of been boxing, drinking or a combination of all 3. Certainly not any evidence to warrant changing the game for the worse
Again though most things in life come with a risk
I'm going to simplify it as much as possible for sake of brevity, but put simply CTE and other similar brain injuries are caused by repeated impacts that cause the brain to bounce off the inside of the skull (i.e. concussions and subconcussive impacts).
BTW, you don't require either contact to the head or a diagnosable concussion (i.e. a concussion with symptoms) for that phenomena to occur. In fact it's pretty conclusively been shown that any heavy contact can cause it. In other words potentially any time a person is tackled no matter how innocuous, and even very light contact to the upper body and head can cause it. Heading the ball in soccer has been proven to cause it for example.
It occurs in many sports that we wouldn't traditionally consider contact sports, admittedly at a much lower rate but still high enough that it's a serious concern, and researchers are as sure as you can be without doing the autopsy that they've even found it in cases where there's been no contact at all. Like in motorsports and bobsleigh for example, where participants aren't getting it just from crashing but from sudden changes in speed, direction, braking, and even just the heavy vibrations.
So no, there's no link between substance abuse and CTE.
The problem you have when studying CTE is that it requires an autopsy to conclusively diagnose and it's symptoms are extremely similar to multiple other condition, like Alzheimer's for example. So we couldn't convulsively diagnose Paul Gallen with CTE unless he's a zombie.
However in saying that, it and other similar conditions prevalence in sportsmen is so high that it'd genuinely be surprising if he didn't have some form brain injury resulting from his participation in contact and combat sports. An NRL player, or any other professional contact sportsman, without some form of brain damage as a result of contact would be a unicorn, and most would have started suffering those conditions long before their professional debut.
There've been multiple studies of pee-wee footballers in the US that have shown that something like (I forget the exact numbers so don't quote me on these) 80% of the ones that played 4 or more years had some form of white matter brain damage (minor brain damage, and a precursor to CTE and other more serious conditions) and somewhere up to 40% likely had some form of grey matter brain damage (that's serious brain damage like CTE)
Here's an article about a recent study (that I haven't read yet) that was the first of it's kind to be able to actually study the brains of 152 deceased athletes that died before their 30th birthday and had their brains donated to science.
63 had CTE, which should put it into perspective of just easy it is to get, and how common it is-
https://www.nytimes.com/interactive/2023/11/16/us/cte-youth-football.html
So yeah, this stuff is super prevalent. The reality is that most people whom played contact sport for any significant amount of time would have some form of brain damage as a result, and I'm not too ashamed to admit that I've been displaying symptoms that could be, and more than likely are, CTE or something similar for quite a while now.
I'm yet to see any professional league introduce rules or protocols that will have any significant impact on reducing the chances of it's occurrence either, and frankly all those rules, head bins, even most stand down periods, seem to exist to make the spectator more comfortable and not to actually do anything of substance to prevent players from developing CTE or other forms of brain damage, and frankly I don't see how you could without fundamentally changing the sports to the point that they're no longer recognisable.