So can a RDBMS... But that doesn't mean that Kafka and databases are related.
As multiple comments have stated above, Kafka is really a distributed message subsystem. Its core interface is a set of topics that one can publish to, and that consumers can read from (in other words, a pub-sub system). Kafka doesn't inspect the message payload at all.
Elasticsearch is a unstructured (to some extent) document store that's optimized around document search. So at the very least, the payload is important when using Elasticsearch.
That's like saying they all write data to disk so they're related.
Elasticsearch is all about saving, inspecting, indexing and retrieving your data through a rich document-based model and search-optimized methods.
ES might be able to do the same thing functionally because it operates at a higher level but ultimately will never scale or be as simple in access as Kafka.
As multiple comments have stated above, Kafka is really a distributed message subsystem. Its core interface is a set of topics that one can publish to, and that consumers can read from (in other words, a pub-sub system). Kafka doesn't inspect the message payload at all.
Elasticsearch is a unstructured (to some extent) document store that's optimized around document search. So at the very least, the payload is important when using Elasticsearch.