Custom index access method for primary keys

Hi! I’m working on an index access method that I intend to be generic and to replace btree for my specific use case. I noticed though that it doesn’t seem to be possible to specify an access method when creating a new table and using PRIMARY KEY.

Same applies to altering a table to add a primary key using an existing index with a non-btree access method. That hits this check and errors: https://github.com/postgres/postgres/blob/4be6a74cfb1ef4c50740e4c2f97a3b478c16de73/src/backend/parser/parse_utilcmd.c#L2480

Wanted to ask if this an intentional limitation and if there is a reason why an index access method (if it supports uniqueness) would not be usable for a primary key?