Replying from this thread:
As far as I can tell, that’s not intentional. It’s indexed, just not optimized. It takes a lot of work to make your content get a high search ranking, especially when there’s lots of other content out there for the same search terms. iNat results are in there, but usually not on the first page. I agree it would be nice to see those ranked higher, though.
For taxon info pages, image alt text/captions are just a small part of it. One thing that likely hurts SEO is having the same content available from multiple URLs, e.g., from each of the international iNaturalist network members. Google would consider those results to be duplicates, which lowers the search ranking for each one. A partial solution for that is to add metadata that indicates which URL is canonical.
Edit: Nevermind, it looks like that’s already done. For the example of Indigofera brachystachya, the first iNat result I see is from portugal.inaturalist.org, which does include:
<link href="http://www.inaturalist.org/taxa/587756-Indigofera-brachystachya" rel="canonical" />
Among other meta tags. So I have no idea why that result shows up instead of the taxon page from inaturalist.org.