Interaction is a cost center in interface design, so a machine-graded Spaced repetition memory system impose greater costs on their users. If the user is meant to type in the answer, that can easily 2-4x the amount of time spent on each task.
Some machine-graded systems dodge the input cost by making their prompts multiple-choice. But this transforms a generation task into a discrimination task—much less effective for memory encoding; see Generation effect