Why can't programmers keep it as simple as dependencies: use system libraries?

The reason is control. If you want your stuff to align neatly and look good in the details you need to define a font. And there is no single system font you can truly rely on, so you just bring your own.

I'd be very happy if OS creators could agree on one set of universally supported system fonts, preferably open source, that work on all systems — until then your two options are:

- not caring about good design, giving up control and using whatever (sometimes this is a feasible option)

- caring about design and using your own font