I understand why you don't like the answers you're getting, let me try a different tone, though I'm afraid my conclusions are the same. Pieces of my background that might interest you? I've been doing martial arts since the 1980s. I chose to give up Muay Thai after a back surgery. I've been married to the same woman for 20 years.
The back surgery? A ruptured disc....from playing softball, by the way.
It's hard to justify why any of us do what we do. I don't know what calls your girlfriend to these sports or why she would expose herself to the pain and risk, but it's something and whether or not she participates, that something will still be there. If it's adrenaline, maybe something like skiing could satisfy it instead...but the worst of my concussions came from skiing. There are people who are afraid to live because everything is dangerous. There are also people are reckless and self-destructive. I doubt that your girlfriend is either.
Maybe she wants to feel empowered. Maybe she's been afraid in the past and doesn't want to live in fear. Maybe she's been a victim or someone close to her has and this is important to her in ways that you don't understand. Maybe she's just a badass. When you date someone for 8 months, part of what you should be doing at this point is finding out if she is someone who you relate to, understand and admire. This thing that she wants to do is something that you don't understand or relate to. Preventing her from doing it is not going to be the foundation for a healthy relationship. Digging deeper to understand why she's drawn to it might. Or she might not be ready to share that with you yet. Or maybe she's not for you, but you can't restrict her activities to make yourself more comfortable and build a healthy relationship with her at the same time.
At my age, with my injury and health history, I have to make some choices as to how I train and with whom. She MIGHT decide at some point that Muay Thai is not the most appropriate way for her to train and there are alternatives, but you can't and shouldn't talk her out of it. I get worrying about her. We all worry about people we love. Listen to the woman, try to understand her, if she's the one, you'll still be peeling the onion 20 years from now. But, support her decisions.