Thread: COPY on partitioned table

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/11/static/ddl-partitioning.html
Description:

"If you want to use COPY to insert data, you'll need to copy into the
correct child table rather than directly into the master."

What the reasons for this requirement? It requires clarification.
COPY to master table will fail? May be performance reasons? May be this
sentence already obsolete?