Most single fruit flavors by themselves do not need steeping. They may change a little, but they taste pretty much what they are going to taste like forever right after a shake.
I'd strongly disagree with you on that one mate
Some juices are, as you say ready straight away, many do need a steep, particularly tobacco and bakery type flavours
Just ask any vanilla custard fan
I have a toffee I mix that gets between 2-4 months steep, it becomes much richer with a steep
Many juices will give a chemical or floral taste if they haven't had time for the flavour molecules to infuse properly
But each to their own, whatever works for any individual is right