Somebody is 'getting ready' they are not yet ready, but are currently doing whatever they need to do to become ready. If somebody 'is ready' then they are already ready.

To be (adjective) = the static state of being (adjective) I am sick. Joe is tired. Susan is ready. To get (adjective) = to change state from not being (adjective) to being (adjective) I am getting sick. Joe is getting tired. Susan is getting ready.

It's just a matter of when in the process we are talking about. If you are still doing things to get ready, say putting on your shirt, for example, ("getting ready" meaning putting on the clothes to go somewhere ) it we say you are getting ready. But after you have put on everything, then you "are ready"