Buy your own damn calculator app and quit whining. I swear, the number of tears shed over this is just silly. There are thousands of calculator apps out there, some of which are very good. If you can't decide, I recommend PCalc - a well-respected and extremely capable calculator with a pedigree going back to 1992.
The iOS calculator app is nothing to write home about. And if Apple just blew it up to iPad size, you wouldn't like it - the keys would be two inches across, on an iPad Pro. So, they'd have to entirely redesign it, to do something with all that extra screen real estate. Which would then bring up the inevitable question, now how do you support all that new functionality on the iPhone? Or do the iPhone users become the new slighted class, the way iPad users have felt for all these years, with a now substandard calculator? This is likely why Apple has never made any big (externally visible) push to bring a calculator to the iPad (other than Numbers). So... get your own calculator and get on with your life.
I agree it didn’t make a lot of sense before, even though as you point out the PCalc dev found a way. But now that they are pushing multi window support I can see them adding it even as a point release to 16 or certainly to 17.
I think Weather would have been more egregious to exclude since they made a bold move buying and shutting down Dark Sky.