So here's what we're actually dealing with. Google maintains the Android Open Source Project, or AOSP. Every handset manufacturer uses this as a base for their own "distribution". The only distributions that Google actually builds are for their own branded handsets and tablets (the Nexus line). All of the other handset manufacturers build their own distribution(s) for their hardware, which effectively makes them the OS vendor for that hardware. It's analogous to other situations in open source software, where, for example, the kernel is developed and maintained by one group, but the individual distributions' maintainers (Debian, Ubuntu, etc) will package/build the kernel for their own distros and release it through their own repositories (ie when I run apt-get on an Ubuntu machine I'm pulling updates from Ubuntu and not, for example, from kernel.org).
This leads to situations like the current one, where the updates have been rolled into new versions (in this case you upgrade 4.3 to 4.4.x) but not every vendor has chosen to build and distribute these newer versions to their customers; Google is no more able to push these updates than the kernel.org maintainers are of pushing new kernels onto your Slackware machine.