From a business perspective it makes sense, to throw all the rendering to the devices to save cost.

Not just to save cost. It's basically OS-agnostic from the user's point of view. The web app works fine in desktop Linux, MacOS, or Windows. In other words, when I'm on Linux I can have a solid user experience on apps that were designed by people who have never thought about Linux in their life.

Meanwhile, porting native programs between OSes often means someone's gotta maintain the libraries that call the right desktop/windowing APIs and behavior between each version of Windows, MacOS, and the windowing systems of Linux, not all of which always work in expected or consistent ways.