Aren't CAs supposed to go through vetting and audits before getting trusted in operating systems and browsers? We even went through a SOC audit just for our own internal CA!
Since many browser vendors are reluctant to remove CAs (because it breaks lots of websites for users), they don't have too much leverage enforcing security practices.
The requirements they are supposed to check against: https://cabforum.org/wp-content/uploads/CA-Browser-Forum-BR-... - section 5 addresses security, referencing "The CA/Browser Forum’s Network and Certificate System Security Requirements" - https://cabforum.org/network-security-requirements/, which at least in the version 1.3 that I saw required pentests and security scans.