There s no fb app in Apple Store for mac

Given that Instagram's iPad app is just the iPhone screen-sized GUI displayed on an iPad with a giant white-space/black-space surrounding it - and I assume WhatsApp is probably the same - my guess would be they simply couldn't be bothered to write apps for systems that can have computer-screen sized web pages open.

They want their apps optimized for mobile users so that they are picking up their phones constantly to check the apps. iPads and computers don't really support that kind of usage.

Probably because Mac users can access it through a web browser, but only the Facebook staff will know for certain.

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Because Facebook didn't make one.

Meta also does not make WhatsApp available in the Mac App Store. It is their choice. You would need to ask Meta why they choose to not do that.