Reddit did not start getting indexed and commonly delivered in generic results for years and years and years. Don't hold your breath. Eventually, though, it'll happen. So long as we're still here and growing.
If you do a search for something like "selfhost site:Lemmy.*" then you get results so the boards are definitely getting indexed. The SEO just isn't there to make it what Google thinks is a better choice when compared to other sites. That may change as these sites get used more and more things are posted and discussed but I wouldn't be surprised if the Google algorithm weighs reddit posts higher than other sites simply because reddit has become such a great resource for things.
What about Lemmy instances which don't have an URL starting with "lemmy"? How will a typical Google user filter results if he's interested in all Lemmy instances? DuckDuckGo to the rescue? 👽
In theory one of the big instances like lemmy.world or lemmy.ml should be indexed and searchable using the site: tag, and their local caches of communities originating on other instances should be included. It might mean that Google returns multiple duplicate pages (one for each instance indexed) however, unless Google takes steps to reduce duplication. Time will tell I guess.
Reddit has orders of magnitude more content than Lemmy. Reddit also has better SEO. I don't think Google is purposely not showing Lemmy content, we just don't have enough content to show.