I am trying to create the following tables in Postgres 13.3:

CREATE TABLE IF NOT EXISTS accounts (
    account_id Integer PRIMARY KEY NOT NULL
);

CREATE TABLE IF NOT EXISTS users (
    user_id Integer PRIMARY KEY NOT NULL,
    account_id Integer NOT NULL REFERENCES accounts(account_id) ON DELETE CASCADE
);

CREATE TABLE IF NOT EXISTS calendars (
    calendar_id Integer PRIMARY KEY NOT NULL,
    user_id Integer NOT NULL,
    account_id Integer NOT NULL,
    FOREIGN KEY (user_id, account_id) REFERENCES users(user_id, account_id) ON DELETE CASCADE
);

But I get the following error when creating the calendars table:

ERROR:  there is no unique constraint matching given keys for referenced table "users"

Which does not make much sense to me since the foreign key contains the user_id which is the PK of the users table and therefore also has a uniqueness constraint. If I add an explicit uniqueness constraint on the combined user_id and account_id like so:

ALTER TABLE users ADD UNIQUE (user_id, account_id);

Then I am able to create the calendars table. This unique constraint seems unnecessary to me as user_id is already unique. Can someone please explain to me what I am missing here?