A major problem is that many deployments of Atlassian products are Server deployments (i.e. not Cloud), which means there's basically the embedded Lucene engine and that's it; anything else starts hurting either performance or deployment options at scale. And most Server customers have a much greater interest in scaling (while keeping servers performant & easy to run) than getting tweaks to search algorithms - in particular many customers refuse to upgrade unless there is a major bug fix or performance boost they're going to get.

On the Cloud side, for better or worse the focus has mostly been on letting people find their work, not find new content they haven't seen before. For example, Confluence's My Work tab (or similar - I forget the exact name), or the search in the cross-product https://start.atlassian.com/ are both useful for getting back to things you've seen before. Hopefully some day we'll finally make some headway on the "I don't know what I'm looking for but I know how to describe my problem" use case.

And just to buck the trend, search works as advertised in the Atlassian product I work on, statuspage.io ;) (but then our search use case is straightforward and rare enough to just use elasticsearch with some minor tweaking).