Feel free to try contributing if you think it that trivial to add "much needed features": https://github.com/microsoft/calculator
https://qalculate.github.io/ https://bitbucket.org/heldercorreia/speedcrunch/
I feel time spent working on projects for a company that hasn't spent the better part of 3 decades trying to dominate and control the entirety of the computer market using strategies that destroy competition and supress innovation, that tracks and monitors its users, that has such a small amount of respect that it will inject ads into a product most customers are forced to purchase should they wish to own a computer, is more valuable in that it makes the world just a slightly better place, rather than a slightly shittier one.
Though I didn't just mean patches from "contribute", it could also mean voice/ideas/discussion: instead of pseudonymously complain on HN, there's Feedback Hub and GitHub Issues. As an option that's available.
Anyway, for instance, you could see that RPN is on the radar: https://github.com/microsoft/calculator/issues/128
I don't think this sort of argument works.
The reason it's open source is most likely because it's got nothing of value to competitors but lets them point and say "see, we've changed, look we make things open source now"; aka marketing.
Fwiw, it seems to go far beyond marketing that Calculator is open source, as community contributions have made it into releases and Calculator has seemed to benefit from more (and more diverse) development resources than it had in XP/Vista/7.