It’s already iPadOS 16.3. Why still no official calculator application for iPad?!

Contributors here are all end-users, just like you. Other than information published by Apple, nobody here has any insight into Apple’s product design decisions or future software development roadmap.

There are some features and native Apps that are only available for iOS (iPhone) or iPadOS (iPad) - the Calculator App being just one of the Apps that are exclusive to iPhone. As for why Apple choose to make some Apps available to one or other platform are known only to Apple.

While the native Calculator App is not available for iPad, there are many free and paid third-party alternatives. One of the very best calculators in the App Store - arguably superior to the Apple Calculator, is completely free and without advertisements - is Sci:Pro Calculator:

https://apps.apple.com/gb/app/sci-pro-calculator/id684978583