Ugh, this has been a huge pet peeve of mine for ages. Not just with the Calculator app, too - it annoys me to no end that there are multiple iOS apps that are not available on the iPad and it just seems so careless. As far as the Calculator app goes, sure, you can use Spotlight, and you can download a third party app in the App Store which may be buggy/ad-ridden/paid, but why should we have to when Apple already codes a very nice Calculator app themselves? Apple is known for generally having elegant software/hardware solutions and this is the opposite of that, IMO.
Your rendering looks great - I’d hope for something like this, only with a regular window option so we can use the scientific calculator and have it side by side and/or in a Slide Over window with other apps.
Seriously, though - all the missing apps should make their way to the iPad. That includes the Phone app (for wifi calling and hopefully voicemail access, somehow), Health and Activity, Weather, and even Wallet, now that people with the Apple Card will use that app to review purchase history and the like.