Client/server models existed before the web and HTML/JavaScript.
WPF is backed by the .NET framework. While the UI framework itself might not be easier than say "React", you have a stable language (C#) with a huge, stable, standard library for all kinds of things from Date manipulation to File IO. There are so many advantages to that ecosystem over JavaScript currently.
For example, I've been using numeraljs for number formatting in JavaScript. This is now un-maintained and contains a bug where if the number is below 1e-6 it shows up as NaN. So now I have to go source a new package for number formatting. This is so simple and core it's amazing I am searching for packages for it.
There is now Intl.NumberFormat but it has quirks also. But this is the crap you have to deal with on a daily basis with JavaScript.