In GA4 BigQuery export data table the ‘entrances’ calculated for (not set) landing pages are usually zero. It is because, for (not set) landing pages, the ‘entrances’ event parameter is either NULL or missing.
The ‘entrances’ event parameter indicates whether a ‘page_view’ is the first page of a session (landing page).
entrances = 1: Indicates the ‘page_view’ is a landing page for the session.
entrances IS NULL or missing: Indicates the ‘page_view’ is not the first page of a session and, therefore, not a landing page. But it could also mean that it is a (not set) landing page.