Atlassian has a separate tool chain, FishEye and Crucible, for code reviews. It's a separate product with a separate license and very few people seem to use it, but the Atlassian suite is meant to be a suite, as in many separate tools for specific purposes, instead of Gitlab's approach of having everything built into the core product. Atlassian's actual code review tooling is just as good as Gitlab's and really better because you can use it across arbitrary repo sources and don't actually need your code to be in Bitbucket or in a single repo or a single server at all.

You're just not using it if you only use Bitbucket.