I have performed reviews on pullrequest.com, and lack of context tends not to be an issue. Code review is an interactive process, where questions can be posed to the PR authors, and PR reviewers have access to search the codebase. Customer success at pullrequest.com also provide context for the repo and organization working practices, attached at the top of PR descriptions, to help reviewers with context and to know what kinds of feedback are useful. Sast and Dast are still not sophisticated enough or widely used enough to be good enough. In-house senior and principal engineers often do not have the time to be on top of every repo and every PR in their organization, and need to outsource an extra pair of eyes. It is highly common that I am reviewing codebases that do not have any Sast or Dast, or have workarounds to silence the tools, resulting in bugs and security issues slipping through. It is very rare that I am able to approve PRs without comments, even after an internal review has taken place!