Same reason your main drive in Windows is "C:" - history.

The command-shift shortcuts (function keys as they were called) were added in the 80's as such:

Command-Shift-1: Eject internal floppy

Command-Shift-2: Eject external floppy

Command-Shift-3: Take screenshot

In the System 7 era in the mid-90's, Command-Shift-4 to select an area to take a screenshot of was added.

Back then you could also install FKEY code resources into the system file to add custom ones, there was a small ecosystem of freeware/shareware function keys with useful shortcuts people would install.