https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=chromium
https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=chrome
https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=firefox
https://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=webkit
Those are the main reasons. And seeing how browsers are essentially designed to run other peoples code with minimal to no vetting processes, I consider this a serious issue.