The integrations for code review and symbol search are both built for internal tools and not amenable to open sourcing.
FWIW I agree that the lack of open source integrations are the main barrier for external adoption
I do wish there was a startup here. There is sourcegraph, which has ok code search (github has come a long way, but without indexing and understanding the build you can’t do it justice). There are also cool code review startups like Graphite, but they don’t work together. I remember how powerful it was to review a change then go use an xref to see how a function is used that is untouched in a code review, so does not show in the diff, which requires checking out the changes locally in OSS land and context switching to leave comments.