One way of thinking of it is that a physical calculator has two parts - the circuitry that performs the calculations and the UI - in the form of physical button

One way of thinking of it is that a physical calculator has two parts - the circuitry that performs the calculations and the UI - in the form of physical buttons, displays, and the connections between them and the calculation circuitry - that allows people to effectively make use of it for their calculating needs. A modern CPU has got the calculation part covered by itself, but it still needs the UI. Of course there are other kinds of UI for people to do calculations - for example Excel, or programming language REPLs - but desk calculators had pretty good UI for some use cases, so it makes sense to have an option based on them.