Because Arrays don't really exist properly in javascript. They're just objects, with a magic 'length' property that is set to the largest positive integer index plus one, and a few convenience methods.
What sort of example do you need to know if an object is an Array? Never come across this problem yet...
Mostly within JavaScript libraries; i.e. for a flatten method.
I agree with you. I guess sometimes it's important to do a type check, but duck typing seems more appropriate for Javascript.