Probably for using HCatalog with can be extremely useful for Pig programmers even if they don't want to use Hive and just leverage this for schema management instead of defining AS clauses in their LOAD commands? Just as likely this is something hard-coded into Ambari? If you really don't want Hive, I bet you can just delete it after installation. For giggles, I stood up an HDFS-only HDP 3.1.0 cluster for https://community.hortonworks.com/questions/245432/is-it-possible-to-install-only-hdfs-on-linux-machi.html?childToView=245544#answer-245544 and just added Pig (required YARN, MR, Tez & ZK, but that makes sense!) and did NOT require Hive to be added as seen below. Good luck and happy Hadooping!
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