Because system resources are cheaper than expert knowledge.

We've got enough RAM & CPU cycles spare that we can run GUIs in a headless browser and CSS & JS have come along far enough to make this easy(ish) to pull off, especially when combined with a framework such as React or Angular.

I can see why it's done, from the cost PoV, but I'm always a little dismayed when I find yet another desktop app which has the 'uncanny valley' of an almost-native look and feel. Same goes for ReactNative vs true native mobile apps.