While techies will know this, here's how Android development works in essence: Google is in charge of developing the base, on which all other customizations worked out by third-parties are put on top of. So what you're seeing on your Samsung Galaxy device is built on top of the code from the Android Open Source Project (AOSP), and the reason manufacturers bother to do this mainly has to do with their belief that it helps them gain a competitive edge.
But while manufacturers skins do bring extra functionality to the table, there are inherent costs involved, as providing support and future updates for a large portfolio of devices, each running on custom code, can be expensive and quite time consuming. As a result, folks who want to have the very latest in Android usually stick to devices with stock or 'vanilla' Android, like the Google Nexus line or even Motorola's bare bones smartphones. A while back, you could still get Samsung, Sony, or HTC hardware with virgin Android running on them, courtesy of the Google Play Edition program, but that has died out and we haven't seen new phones join the ranks in a long while.
It's not just about the speed of the update process, though, as stability and performance are (typically) impacted negatively whenever a third-party builds on top of Android. Sure, there are (very rare) exceptions to this rule, but it's generally the way it is. With that, and the above, in mind, would you have preferred it if your smartphone came with stock Android instead of a manufacturer skin?
