When something is unlikely to happen, difficult to do or hasn't happened, you can compare it something more likely/easy (but still not possible) or something that hasn't happened, to emphasise it. For example: I have never left the city, let alone been abroad. I am too tired to walk, let alone run. I won't even be able to finish the race, let alone win it.

merriam-webster.com: : to say nothing of : not to mention —used especially to emphasize the improbability of a contrasting example <he would never walk again let alone play golf — Sports Illus.> <how many ever see an Ambassador or Minister, let alone a President — Robert Lacville>