As noted in the documentation, serials are not "real" data types, but rather convenience wrappers. If you create a serial column, you automatically get

• a new sequence ('tablename_columnname_seq`)
• an integer column of the appropriate type that takes its default value from the sequence.
• the setup for the column to use the sequence.

To quote:

The data types smallserial, serial and bigserial are not true types, but merely a notational convenience for creating unique identifier columns (similar to the AUTO_INCREMENT property supported by some other databases).

CREATE TABLE table (BIGSERIAL id PRIMARY KEY);

is the same as

CREATE SEQUENCE table_id_seq;
CREATE TABLE table (
   id bigint NOT NULL DEFAULT nextval('table_id_seq')
);
ALTER SEQUENCE table_id_seq OWNED BY table.id;

Which matches what you are getting.