Why am I not able to install WhatsApp on iPad?

Meta has no interest in launching an app for the iPad. That is not Apple's problem and by default all iPhone apps will work on an iPad, so Meta has to specifically restrict access to the iPhone only so it does not run on the iPad. You seem to think that it is Apple that is stopping them from doing it and you would be wrong.

Meta has been asked for years to allow WhatsApp on the iPad, but to be frank, they are only interested in a device that has a phone number. Get in line and complain to Meta. As with any device that does not have a phone number, such as the Galaxy Tab, Mac computer, Windows PC and iPad, the only way to use the web version of WhatsApp is to link it to a device that has a phone number.

Yours is a question that can only be answered by the WhatsApp Developer/Service Provider. This is not a limitation of iPad or Apple.

The WhatApp developers have not released an App compatible with iPad - the App being currently limited to iPhone. Whilst the App is not available for iPad - you should be able to access the service using a web browser:

https://web.whatsapp.com/

You should note that to use WhatsApp, each user requires a phone number - as this is associated with your WhatsApp account.