Oracle Update Statistics Partitioned Tables

Let me first address why it updates the stats for the entire table: because that's what it did before they added table partitioning.
Changing the default behavior of a command is very difficult to get approved because it breaks backward compatibility. Somebody somewhere may expect it to work the way it always worked for them before. To be honest, who of us has not complained when something changed it's default behavior? This is why we still have features like auto-shrink, because somebody somewhere may rely on it.
Incremental stats is the feature you want to use here, and it would not be difficult to write a script to determine which partitions need to be updated.
Some good related info here: https://blogs.technet.microsoft.com/dataplatform/2016/04/03/incremental-statistics-how-to-update-sta...
Oracle update statistics partitioned tables 2017

Oracle Database Statistics

Oracle: DBMS_STATS Gather Statistics of Schema, Tables, Indexes. DBMS_STATS package, which lets you collect statistics in parallel, collect global statistics for partitioned objects, and fine tune your statistics collection in other ways. This package is concerned with optimizer statistics only. Update statistics supports partitioned tables. For more information, see SAP Note 424243. To increase the accuracy of statistics for partitioned tables, the sample size is doubled. BRCONNECT support for system and Oracle dictionary statistics.

Oracle Update Statistics Partitioned TablesOracle Update Statistics Partitioned Tables

Oracle Update Statistics

“Hidden” Efficiencies of Non-Partitioned Indexes on Partitioned Tables Part II. Internals, Local Indexes, Oracle Indexes, Partitioned Indexes. Index Compression Index Internals Index Organized Tables Index Rebuild Index statistics Oracle Cost Based Optimizer Oracle General Oracle Indexes Oracle Index Seminar Oracle Myths Oracle. Gathering Statistics on Partitioned tables. Gathering statistics on partitioned tables consists of gathering statistics at both the table level and partition level. Prior to Oracle Database 11g, adding a new partition or modifying data in a few partitions required scanning the entire table to refresh table-level statistics.