Why Microsoft doesn't just focus on a single product and fix all of the security holes in one shot?

Finding bugs in products doesn't work that way. Every product that Microsoft codes goes under dozens of manual and automated tool reviews. That scrutiny is vital because Microsoft is the biggest target, and as a result Microsoft products actually have fewer vulnerabilities than those of its nearest competitors. But even with the right tools and processes, you can't catch everything. New techniques are found, mistakes are made, and until you have perfect humans, you'll never have perfect code and you'll never have perfect bug detecting.

Source : http://technoeck.blogspot.com/2014/01/13-tough-questions-about-computer.html