They're also not HiDPI/Retina, and the whole website design seems to be stuck in the mid-2000s, too. Somehow mobile and web developers got aboard all the new design and interaction trends, but Desktop GUI frameworks seem to be all stuck 10 years behind.
If I had to guess, I'd say it's probably because of a lack of capital investment in desktop software.
And yet a lot of people don't understand why everyone is using Electron for desktop apps these days.
Because the new design trends are all about how to adapt to the limitations (and possibilities) of mobile and web. For desktop applications these mobile-first designs don't work very well (see Windows 8) and we already had long-established design patterns that do.
Either way, at least in my experience, I had to look up the C++ documentation because the Python binding documentation was never complete.
I have been using wxpython on and off for last a few years, the nice thing is it gives a natural looks on each supported OS, so your application won't standout as odd/fancy/weird/off, as with some other cross platform GUI framework do;
Compare these pages...
* Overview, https://www.wxpython.org/pages/overview/
* API, https://docs.wxpython.org/
* Demo, https://wiki.wxpython.org/WxSmallApp
...with these pages for Bootstrap:
* Overview, http://getbootstrap.com/
* API docs, http://getbootstrap.com/docs/4.0/getting-started/introductio...
* Demo, http://getbootstrap.com/docs/4.0/examples/
For me, this difference made web UI frameworks much more approachable. Visual widget catalogs simplify the task of finding widgets or code samples. Widget screen shots help determine if code is performing as expected.