I actually heard something about that in class not long ago
The story is that Android's security heavily relies on the compartmentalization of apps that lives in the android layer, over the Linux kernel. Apparently, that functionality works in part because only this layer can perform operations that require root access, no app or user can.
So software that allows you to root your phone apparently breaks this requirement, and makes the whole OS insecure. He even heavily implied that one should never root their phone with 'free' software found on the internet because that was usually a front for some nefarious shit regarding your data.
I'm just parroting a half-understood and half-remebered speech from a security expert. His credentials were impressive but I have no ability to judge that critically, if anyone knows more about this feel free to correct me.