What about DynamoDB? I'm pretty sure they could open source a lot but they just don't want. Google had the same pathetic excuse when asked why they don't open source appengine or datastore.
Releasing stuff isn't simple, the hosted services only work because of the rest of their infrastructure. Writing about the tech is usually the best way to spur clean open-source versions that we can all use.
From what I recall DynamoDB is a somewhat hacky API layer over a bunch of MySQL clusters. I'm not sure anyone would, or should, attempt to use a non-managed version of it.
The recent MySQL info comes from this 2016 thread on DynamoDB storing empty strings: https://news.ycombinator.com/item?id=13170746
Confirmed by this comment specifically: https://news.ycombinator.com/item?id=13173927
The latest 2018 ReInvent deep dive on DynamoDB doesn't reveal anything but still fits if mysql is powering the storage nodes: https://www.youtube.com/watch?v=yvBR71D0nAQ
But there is more to this as well. AWS is a huge ecosystem by itself, a lot their services intertwined with each other, simply open source DynamoDB isn't enough without open sourcing the networking/storage functionality that is probably provided by other services.