Screenshots are made on Windows 8.1 box, the windows chrome comes from there.
Plus the whole thing is meant to work on ancient Windows versions (like, Vista and WS2008 ancient), so that ultimately defines the minimal common UI denominator.
Let me chime in and say that plain Win32 API is a perfectly viable option if you are using C++ (or another "OO" language) and if you are willing to sink a couple of weeks into writing your own MFC-like wrapper.
Clearly this is not an option for those who are just starting up with Windows GUI work, but with little experience it is really a matter of 2-3 weeks of ground work and then you have full control over all nuances of the UI, yours to extend and mend as you wish.
If there's one thing that Microsoft is really good at, it's ensuring deep backward compatibility. So anything that's based on Win32 API is going to be stable. If it works now, it will work later.
I have some examples from 10+ years of development updates accumulated here - https://bvckup2.com/wip
Did an AB test a while back and mere presence of the PayPal option boosts sales through all other options plus generates extra sales through PayPal itself. Not something that I was expecting at all, but the confidence level was over 99%.
Plus the whole thing is meant to work on ancient Windows versions (like, Vista and WS2008 ancient), so that ultimately defines the minimal common UI denominator.