Perhaps another reason for this is that Javascript is inherently unsafe. By relying on these small rigorously tested libraries they are avoiding the need to test their own code, and thus avoiding basic null check or conversion errors.
Other languages handle this with compilers, may be strongly typed, and/or have features that don't allow nulls. Javascript doesn't exactly have that luxury. So maybe it makes sense in Javascript, where it wouldn't in other languages (though one could argue this is a flaw in the language design).
edit: to be clear I'm not really defending the practice, but trying to give a little perspective.
Other languages handle this with compilers, may be strongly typed, and/or have features that don't allow nulls. Javascript doesn't exactly have that luxury. So maybe it makes sense in Javascript, where it wouldn't in other languages (though one could argue this is a flaw in the language design).
edit: to be clear I'm not really defending the practice, but trying to give a little perspective.