The reason is simple: it only supports exact-matching text search _on titles_. There are a host of other usability problems that are so bad that they render a giant negative impact on information retrieval and actively steer Q&A traffic to repetitious and ephemeral tools like Slack.
But the lack of a functional search engine indexing the Q&A content with a high-performing notion of search relevancy is such a show-stopping, egregious defect that even all the other egregious flaws can be ignored by comparison.
It’s not a subjective debate at that point: just a factual defect of such severity that any misguided reports of the tool somehow miraculously being even marginally useful for someone have to be ignored.
I thank my lucky stars that Stack Overflow offers competition to this with a real search engine.
[0]: < https://community.atlassian.com/t5/Questions-for-Confluence/... >
I am on a committee in my workplace that deals directly with Confluence customer support to roll out changes to our internally deployed Confluence & Jira instances, and I know first hand from dealing directly with Confluence that they _intentionally_ only support exact matching _on the titles_ for searching in Questions. Really appalling.
Jira search is useless, you have to use filters and memory of dates to get a set of issues small enough for you to search manually.
The search in Confluence can best be summed up as "Here's a PDF". For some weird reason Confluence can search PDF sort of okay. It's typically not what you want, but because Confluence can search PDFs better than normal Confluence pages, PDFs are what you get in 75% of your searches.
Atlassians continued failure to deliver a half decent search feature in any of their products is fascinating. I honestly don't mind their products, they're fine for our needs, except you can't depend on search.
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).
For a smaller company though, other than as a search tool (previously asked questions) I don't see what advantage it has over just using Slack/Teams etc. in a relevant channel to your question.
Disclaimer: long time Atlassian employee but all words are my own.
As I became responsible for an internal library used company-wide, I pushed for the Q&A site to be our primary means of communication, and would always refer to it after answering questions from people contacting me directly. I saw the same pattern repeat itself for many new hires.
Otherwise I think your observations are spot on.