A lot of people have adopted the Elastic stack over the years, I guess Amazon just wanted their share.
The Elasticsearch Service has problems: you can't join the cluster like you can with the normal version, permissions are janky at best, it's slow, and it's expensive. For those reasons the projects my team used it on opted to roll our own elasticsearch cluster which proved a better solution long term beyond the initial annoyances. I say that to say it's not a one to one product, which will probably be their defense.
But they usually tend to build their own alternative that integrates better into their eco-system.