It is weird that I have difficulties to understand this simple passage

(1) What does " the light of day outnumber the sand grains of Arabia"? "Photons of day outnumber the sand grains" sounds nonsensical to me.
Does it refer to "good days outnumber bad days"?

(2) "Certainly those unborn ghosts include greater poets than Keats, scientists greater than Newton"? Keats was an unborn ghost? He just passed away and actually never became a ghost?Scientists greater than Newton are very few; Einstein was one, Darwin may be one of them if you don't mind. Who else?

(3)"We know this because the set of possible people allowed by our DNA so massively exceeds the set of actual people"?
Theoretically every pair of sperm and ovum can grow to be a fetus. But millions of sperm die in the competition for an available ovum. Is this what the author refers to?

(4) "that prior state from which the vast majority have never stirred"? The vast majority have never stirred the prior state yet we have the right to stir it/get our life started?

Context:

“We are going to die, and that makes us the lucky ones. Most people are never going to die because they are never going to be born. The potential people who could have been here in my place but who will in fact never see the light of day outnumber the sand grains of Arabia. Certainly those unborn ghosts include greater poets than Keats, scientists greater than Newton. We know this because the set of possible people allowed by our DNA so massively exceeds the set of actual people. In the teeth of these stupefying odds it is you and I, in our ordinariness, that are here.We privileged few, who won the lottery of birth against all odds, how dare we whine at our inevitable return to that prior state from which the vast majority have never stirred?”
― Richard Dawkins, Unweaving the Rainbow: Science, Delusion and the Appetite for Wonder