Because I can. Turns out you can change the default editor font of TinyMCE and if you so desire make it match the font seen by visitors to your website. You create a special css file place it in your theme and then add a function to functions.php file to load it.
It’s not something that’s ever been on my radar. Much of the time I’m editing in the Text tab (so I can see the raw HTML) and whether I used the Visual tab I always used the Preview button to see what my changes were going to look like, but I can understand why someone might want to change the default font of their editor.
I always learn new stuff from client requests.