Your welcome 
To be honest, I have created my own solution for not relying on the font field in themes.
For custom themes, I don’t use the font field as long as the client doesn’t use Google Fonts.
Simple answer is: Because of UX. As the font is something that doesn’t get changed that frequently and is a major setting, sometimes less options are better to not let the design go down.
Something that I’m always providing tho, are all major font-sizes(h1-h6 for all breakpoints, buttons and other things that might be in the custom theme). Doing so the client/user can always fine-tune them. without messing to much.
Also I’m always deactivating the default rich-text and provide a custom one without font-family, font-colors and such. It’s way more work for sure, but personally I don’t want to see something like Arial/Verdana in combination with an actual font from Adobe, MyFonts etc.