Anki, Mnemosyne, and SuperMemo are self-graded. Duolingo, Memrise, and Quizlet (for the most part) are machine-graded. The Spaced repetition memory system products with widespread adoption and significant commercial success are all machine-graded.
I had initially thought that might be because the commercial products’ revenue models all rely on selling to teachers, but that isn’t true: all three seem to draw their revenues primarily from users (though Duolingo and Memrise are not profitable as of early 2020).
I feel instinctively that there’s some relationship here with The dominant culture around spaced repetition memory systems is fixated on meaningless goals: the marketing around these tools centers very much around “achieving learning outcomes,” and that creates an expectation (probably on both sides) of “getting a grade,” seeing accuracy numbers go up, etc.