This is deliberate. Jobs saw what the calculator app looked like on an iPad and pulled it at the last minute.
This is a problem specifically caused by the iPad's design. On desktop you just prohibit the user from making the calculator app window bigger than reasonable and that's that. But iPads require that the current app fill the entire screen, so any apps that can't are really awkward and frustrating to use - as can be seen when you run an iPhone-only app on an iPad[0]. Third-party calculator apps have the same problem, too.
Before you mention Stage Manager, I should point out that making the calculator app an iPad Pro (2021) exclusive feature would be even sillier than the current situation of not having one.
At one point this was solvable by putting a widget in the Today view, which would mean you had a properly-proportioned calculator on your homescreen. However, several iPadOS updates have broken this fix:
* iOS 14 introduced a new widget type intended to live on homescreens that does not support user interaction. In iPadOS 14, these widgets sat on top of the older interactive ones and often pushed them far off the bottom where they were basically useless.
* iPadOS 15 redesigned the homescreen to bring it to parity with iOS 14[1], at the expense of completely removing the ability to have the Today view on the homescreen. You now have to swipe to the left to get at it.
So on a current-OS iPad, the only place for a calculator app to live that is guaranteed to always be a reasonable size is the legacy widgets section of a rarely-used Minus World of a homescreen that lives to the left of your other homescreens. Everywhere else incurs various levels of silly-looking design.
[0] Only recently did we even get the ability to rotate iPhone apps contrary to the device orientation. Before that, if you used a keyboard case, iPhone apps would more or less demand you take the iPad out of the case and hold the iPad in portrait in order to use it.
[1] If I had a nickel for every iOS feature that took a year to come to the iPad, I'd have two nickels.
Space is not a good reason: you could easily fill some with a calculation history log, or even implement type/editor features like the Numi app on macOS.
