We do have libraries which do offer some features that are not easily available. LibAddonMenu for example is used in over 400 addons and offers a common interface to offer settings for players. But as Ayantir said, there is far too few addon authors to do this for every single part of the game's API and it would in my opinion be a waste of time to reimplement the whole API. Why should we build a second one on top of the existing one when most methods are stable anyways?
It's also not as if ZOS just removes or renames or changes the methods all the time. They even offer
addoncompatibilityaliases.lua which helps old addons to stay compatible with the newest API when something does get moved. But there are some changes which simply cannot be abstracted away by a library or aliases. For example the removal of veteran ranks. There is simply no way you can handle such a big change without updating addons which use that information.
Besides, broken addons are not caused soley due to changes made by ZOS. Quite often it's because the addon author did something like overwriting a part of the UI, relying on a certain structure in the UI controls, using some undefined behavior of a feature in the API or incorrectly calls some method. (I am guilty of at least two of these things
)