They don't.
There is nothing stopping developers from selling MacOS apps outside of the App Store. It's the developer's choice.
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They don't either.
People, stop making up non-existing problems.
The App Store is an optional market that developers can freely take or leave.
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"notarization" is an unfortunate, and confusing name. In the U.S. at least, that word refers to having a signed document witnessed and attested by a government-enrolled agent (a Notary Public) who witnesses signatures for a small fee. I've *never* seen the word used in a different context before.
My mother was a Notary Public. I used to play with the embosser when I was a little kid. Shhhhhhh!

(Imagine Bart Simpson let loose with a Notary embosser! I didn't do any of the evil things he'd do with it - just stamp random blank sheets of paper.)
Can't techies stop overlaying the common usage of words with their own specialized meanings?