Why Are Not All Of My Global Tabs Being Displayed Under The Product Edit Screen?

It’s probably because you have cloned/drafted a global tab from the Global Tabs table listing. When you clone, the unique field ID of the tab remains the same and as it is no longer unique it will fail to be listed in the Global Tabs on the product edit screen. To make sure a new (unique) Field ID is generated, always use the “Add new” button to create new tabs.