Usually all one wants to do is extend the existing rendering with a slight behavioural tweak but this is almost always made impossible by the way Xamarin has designed the standard library of renderers. The key methods are always either internal or private.
Take the DatePicker for example. A common and expected use case of this renderer would be to allow customisation of the DatePickerDialog (Android) that is used under the hood. Why is the OnTextFieldClicked method not a made at least protected virtual so we can override it? Giving it a more suitable name would also be a good idea.
I think Xamarin forgot when implementing the stdlib of renderers that we might actually want to just enhance them rather than write our own from scratch which is actually a very rare use case for most Forms developers.
Tragically it's not that easy to fix this now without doing a painful audit of each renderer implementation.