You obviously don't write software for a living. It takes effort to redirect people to an unmaintained code base and have them both write and investigate possible side-effects of their patch and then deploy it in a format that's usable by all the manufacturers with devices out there. Its an actual cost to an actual company doing actual business that just isn't worthwhile.
Being an open OS, there's nothing stopping Motorola, Samsung or LG from patching their own versions of 4.3 either, just as they modified it with their UI and other extensions. Feel free to whine to them instead; unless you bought a Nexus device, they sold you the phone, Google didn't.