Perhaps because they would like to call into code written in Go.
No Kafka Streams for Go yet.
re: yet
Is the work in progress? If so, please tell us where.
As I remember, they said that tgey want to stabilize API before adding support for new languages
Goka is not a replacement for Kafka Streams, they can be used together. For example, you can use streams and tables created with Kafka Streams inside services built with Goka.