Why aren't the Jailbreak tweaks built into iOS by Apple themselves?

Its simply a nobrainer for Apple to include at least 90% of UI tweaks available at Cydia.

A simple tweak like SwipeSelection Pro has the ability of making iOS Keyboard so very intelligent.

What I fail to understand is, what is holding Apple to include these awesome tweaks into iOS directly? Is it due to performance hit or copyright/Patent violations or something else? What is holding them up?