That way you're not having to completely redo your search infrastructure at some point in time.
You can seamlessly scale from 1 document to billions without any change to your code, backup approach, monitoring etc.
Sorry, I am not sure why you brought up a single instance. If you run on a single instance, you lose half of the benefits of ES which is trying to be highly available.
If we have an index which does not need any backups, single instance is fine, but for most people single instance is not an option. Furthermore, if one is already struggling with 3-4 nodes, how helpful is single instance?
My point was merely that ElasticSearch scales from very small use cases to very large ones. You don't have to setup a cluster. And that using your SQL database isn't going to be significantly easier to manage.
A SQL database doesn't run out of memory and crash when you add documents to it "too fast". ElasticSearch does.