That’s too broad a statement. Can you substantiate why you think so?

The book below provides a nice overview of using Java for DS and ML

https://learning.oreilly.com/library/view/data-science-with/...

Seems a bit of an overstatement - there are huge production systems doing massive ML using Spark, Hadoop, Kafka etc. In some areas these are the defacto solutions. They can't all be "not in their right mind".

Unless there were doing fairly simple ML that needed to be tightly integrated with a pre-existing Java codebase.

Can you elaborate on why you feel that way?

I will risk that I get downvoted to hell. The data scientists in my company and pure geniuses. But. They can barely program. I wouldn’t trust them with Java, I’m sure they wouldn’t trust themselves either. Python is slow but at least it’s easy and fun. I don’t see anything else getting traction for long years.