Because it is easier to embed it into a browser rather than persuade vendors of all major OSes (Windows, Mac, Android and thousand of Linux distributions) to ad

Because it is easier to embed it into a browser rather than persuade vendors of all major OSes (Windows, Mac, Android and thousand of Linux distributions) to add it. Also, even if a company like Microsoft adds it to Windows, they will add it only to they latest version and leave people on Windows XP, 7 and 8 without protection. Same with Google - they will add it only to the latest Android. Because commercial companies want you to buy new products, not to use the old one for a long time.