You said it yourself 'vary depending on the setting'. But this isn't Eberron here, this is Forgotten Realms. In this setting divine magic comes from gods, and the very backstory of the BG Saga-the Time of Troubles is one in which the consequences for worshippers whose gods are killed or depowered is spelled out in no uncertain terms (nor is it the last time in the setting-it's a reoccurring major plot point). So while deities might be optional for paladins (and clerics, if we are talking 5e), that doesn't mean it is an option that is necessarily appropriate for a setting such as this. And certainly not for it to be the default.
