Even if we say, 80% of those are scammy ad/malware crap, that leaves 16k. (I'm using totally guessed numbers, so I'll say that while Sturgeon's law says 90% will be crap I'm deliberately including the crap that isn't scammy)
If most of those get revised and improved, cool. If not...that's a lot of frustrated potential developers.
I don’t have any apps there, but from what I hear the Mac App Store is almost entirely a different ballgame.
Developing apps is a big investment in time (if nothing else). It's hard to imagine you could get to the point of having any kind of reasonable app to submit without overcoming a few roadblocks to get that far, so what's one more?
I suppose I don't have a good feel for the proportion of shovelware being submitted, where apps submitted is very low effort, and a "developer" would be fine with walking away from it. If there's a lot that would skew the numbers.
"would be" and "could be" are not the same. Note I'm not trying to "blame" (or justify) Apple here - just trying to get a grasp on how many people try and...stop (or are stopped).
The second-place item is “privacy” rejections, which surely lead to many more developers abandoning their ideas. (Good.)
Apple did not provide enough data for us to answer your question with any more certainty, but at least they offered enough to guess with.