It's extra resources, time, and complexity spent on forcing something on the visitor that in most situations they don't want or care about.
Why not just let system designers choose the fonts their users are subjected to by default.
At least they seem to care more about accessibility than XYZ.coms marketing department.
* I don’t know there’s a config at all
* I know there’s a config, but I don’t know enough about fonts to make a good choice
* Most sites override my config anyways (unless I chose an even more aggressive config that breaks websites), so why would I care?
* The idea of some site specifying a font does not bother me, even if it’s a web font that will cause an extra 25kb of download.
For what it’s worth, I really like fonts and know my way around the Firefox config, but I honestly do not care enough to change the default font because of the last two items on that list.
* I know there's a config, but this is my work machine and corporate IT has blocked the browser prefs dialog via group policy
It's unfortunate Courier New is both so badly different from the regular fonts (and so bad at all, by the way) and also the default on Windows.
Using Courier New for code samples is not a great idea. I think it comes from the times when good monospace fonts did not yet exist. I always change this default, andvi wish browsers somehow stopped clinging to it.
(The "font-display" setting is also a setting that the user might want to change even if font downloading is enabled. My idea of "meta-CSS" would give you this and many other things for free. Meta-CSS would not inherently allow the timing to be adjusted, but it is a feature that can be added too if it is wanted.)
Another thing that I might want is to enable the document to specify its own fonts if it is SVG or PDF, but to disallow HTML documents from specifying their own fonts.
I very much doubt that's true for the average user, who will simply be using the browser's built-in default – which to an not inconsiderable extent is rather constrained by backwards compatibility concerns rather than being truly optimised for usage as "the only stylesheet you'll ever need".
Besides, even with a boring plain old homepage-and-definitively-not-a-web-app it's not too difficult to run into the limitations of what you can achieve without some custom styling even purely in terms of basic layouting and typography.