What's so magic about JDBC batch udpate that you can't do in a query alone?

A batch processing task cannot be replaced by a query. And, I'm sure you know the effect of changing large volumes of data in a single query, like locks held for too long, lock escalation, VACCUM delays, and impact on undo logs, right?

How does every other technology do that?

All the good data access frameworks use the JDBC support for batching that was added in 1998. The addBatch and executeBatch methods of the PreparedStatament do all the magic.

Why you can't just send 100 queries separated by semicolon which I assume "JDBC batch updates" does?

Because I wouldn't want to risk SQL injection attacks, would I? Concatenating SQL statements does not work for bind parameter values. Your solution implies injecting the parameters in the String statement and then concatenating those and sending them in a single JDBC Statement. No serious framework or developer would ever do that.