I am a Windows power user, and the default Windows calculator is one of very few programs bundled with the OS that I actually use - and I use it quite frequently.
And I click on the number digits.
Why?
1. Usually I only click on a few buttons, for example I paste some number, then press / then 2, and then typically ENTER key on the numpad (because I can just press it with the thumb of the hand still holding the mouse). Why should I move my hand from the mouse to the keyboard in order to just press two keys? And then I would also need to check if Num Lock is enabled.
2. I had a phase trying Emacs and Vim and eventually decided I don't care about being more efficient at text editing. I want to edit the text effortlessly, and I find selecting text with mouse, or pressing arrow keys repeatedly, actually requiring less effort from me. Yes, it takes more time, but it's not a time lost, I'm thinking about some problem, and no part of my brain needs to think what kind of text editing function I should use. Likewise, clicking a few buttons in a calculator takes more time, but it's not a race. If I have to make a lot of calculations, I'm more likely to use Excel, so it's rare that I actually type stuff into the calculator.
3. Sometimes my left hand is simply not available (I'm petting my dog or eating something or maybe holding my laptop), and while I have a numpad, using functions like sine would require my right hand to travel. In similar vein, sometimes instead of Ctrl+clicking on a link to open it in a new tab, I just right-click, and choose a context option. It does annoy me slightly, but sometimes not enough to start using my left hand.