While CI is integrated, it does not force you to use it, and there are ways to integrate with external solutions.
While CI is part of the same product, you don't run the tests in the same machine, so you still need to provide the workers in additional ones or use the autoscalling mechanisms and have it bootstrap machines in the cloud as needed.
So what I mean here is that while it is part of the product, it's mostly the "frontend" and the "APIs", the heavyload part of running it is totally optional.
But there are definitely some customers who are sidelined by this approach. Atlassian is pretty committed to tools that talk to each other with documented APIs (Jira, Confluence, Bamboo, Bitbucket/Stash), and for large organizations, that's often a better fit.
There are many players doing "unix" application, and very few trying to build a suite. You need to pick your fights.
Basically, they build the hooks but aren't dogfooding any of it, since their own integrations don't have to.
And speaking from the perspective of someone using GitLab with Jira and Jenkins, it just isn't the same. Is that necessarily because of problems with the interface or is it just those specific plugins? I don't actually know.