[snide]

Primarily because they focus on interfaces, not publishing. Serif is traditionally used for legibility of long text. Sans always wins for actual interface design (at least those with some manner of complexity) because it's much less visually distracting when used in a variety of weights and sizes. Interfaces tend to have much more mixed usage so we usually go with sans.

Generally just using the system font is fine for most UI interfaces and the only good reasons to change it are:

1. You have a specific branding style you need to achieve (this is becoming rare) 2. Your interface has pixel perfect user created content where the difference in font to font usage between OSes / Browsers would cause actual breaks in layouts from user to user.

I run into the later often, and so we end up picking a neutral style font like this because it's the closest thing to a generic "system font" like Roboto or San Francisco that has a friendlier license.

Personally I prefer Inter UI https://rsms.me/inter/

[airstrike]

Thanks for the link! I've been looking for a free-as-in-beer, good-looking font family with tabular numbers and had pretty much given up on hope. Inter UI is fantastic!

[pidg]

Ooh, Inter is very nice. Thanks!