Why does spark.sql MERGE INTO not work with streaming sources?

I'm trying to do the below execution in Spark 3.1.2 / Scala 2.12:

streamDF.createOrReplaceTempView("tempStream")
val query = spark.sql(
  s"""
     |MERGE INTO catalog.db.tablename t
     |USING tempStream s
     |ON t.ID = s.ID
     | WHEN MATCHED THEN UPDATE SET *
     | WHEN NOT MATCHED THEN INSERT *
     |""".stripMargin
)
query
  .writeStream
  .format("console")
  .outputMode(OutputMode.Update)
  .start
  .awaitTermination

But I'm getting this exception:

Queries with streaming sources must be executed with writeStream.start()

However, the below execution works well:

streamDF.createOrReplaceTempView("tempStream")
val query = spark.sql(
  s"""
     |SELECT COUNT(*) FROM tempStream
     |""".stripMargin
)
query
  .writeStream
  .format("console")
  .outputMode(OutputMode.Update)
  .start
  .awaitTermination

Can someone please explain why is this happening? Is there some alternative for MERGE INTO in this case?