[exikyut]

Windows/macOS/Linux aren't the operating system any more, the browser is.

And the browser's job is to be constantly online the whole time and download and execute JavaScript that gets dynamically optimized for your CPU architecture using one of the fastest runtime compilers ever made (aNd WhiCh MiGhT HaVe BuGs iN iT), and then your CPU directly, blindly executes the result, with as little bounds-checking as the runtime compiler thinks it can get away with so it runs as fast as possible.

Zooming out somewhat, the new OS paradigm is the continuous download and execution of absolutely arbitrary code, all day, every day, from sources including hacked ad servers, successful social engineering campaigns and your blog.

And Chrome has like ~70% market share.

Because public company and "legally bound to create value for shareholders" and all that, it is very much in Google's interest that they maintain that market share because that lets them serve more ads.

So that's ultimately the reason. Google wants the world's most secure platform so they can guarantee their ads business.