Org admins can't hide offline sandboxes on the Apps → Sandboxes page. These are sandboxes that were deactivated, or that went offline after 28 days without use. They stay in the table with the status DELETING until they're permanently deleted 30 days after going offline.

Organisations on Enterprise plans can have up to five sandboxes per app
For them, the list can fill up with sandboxes they've already chosen to remove, which makes the active ones harder to find.