The software ecosystem is wet
blog.alexrohde.com
blog.alexrohde.com
even kafka (basically just log part of mysql without columns). For example, a kafka topic could be seen as a table with the columns: offset, value.
In much the same way that a block of carbon with a couple gas cylinders (H2, N2, O2) on top is "basically just" a person...Kafka is very basic tool in comparison, that drops 90% of functionality for a speed gain in one use case.
There's no reason you couldn't write a custom storage engine that behaved exactly like kafka with Postgres.