MySQL (SQL) | LEFT JOIN DOES NOT RETURN ALL ROWS (NO FILTER APPLIED)

I would like to left join a calendar table with a sales table and return all sales by user by day (inclusive of all days without sales). For some reason, however, I only get the days with sales. As you can see below, there are no filters applied. What am I doing wrong here?

Below my SQL code, all tables and the wished final result.

MySQL Query

SELECT
  calendar.date AS date,
  sales.user_id AS user_id,
  CASE
    WHEN sales.date = calendar.date THEN sales.quantity
    ELSE 0
  END AS quantity
FROM calendar
LEFT JOIN sales
  ON calendar.date = sales.date

DATE table (from 2020-01-01 to 2020-01-31)

SALES table

WISHED RESULT

The user is also unknown for the day without sales. To solve this, you can do something like this:

SELECT
  calendar.date AS date,
  su.user_id AS user_id,
  COALESCE( sales.quantity,0) AS quantity
FROM calendar
CROSS JOIN (SELECT DISTINCT user_id FROM sales) su
LEFT JOIN sales
  ON calendar.date = sales.date AND sales.user_id=su.user_id
ORDER BY 2,1

see fiddle