Why do companies talk about paying for CPM ???

Why do companies even bother to say they are paying based on CPM when it seems to me they all adjust the rate down based on click rate?
Why can’t they just be honest and up front about it and admit they are paying a click rate, not an impression rate?
Throwing words out like "Smart Pricing" only makes it worse the way I see it
anyone care to explain why they do it this way, am I missing something?

Original post by ruffnat