Because it would limit all the 8.x updates. And it would be quicker to deploy the update via app store vs the OS. Plus I dont want a bloated OS. Its no different what Samsung does with stock Google. Motorola learned to decouple their core apps and there has been no complaints with this practice.
Apart from the fact that the OS works totally different, so it would waste time doing that, which could be better spent doing something more worthwhile.
Additionally, separating the apps out, isn't going to buy you much more speed deploying the OS updates, especially on delta updates.
Again, you miss the fact that a lot of the core apps will lose functionality if they're separated out from the OS. Which will probably have you complaining more.