After the first primates developed grooming, we've seen how our ancestors began using vision rather than their other senses.
This has resulted in an over-reliance on that sense.
This tendency had adverse effects on their grooming technique because, as explained in the mirror usage section;
•Folds can hardly be seen,
•If you use your sense of sight, it overwhelms your weak sense of touch.
All the grooming procedures can be performed at night, with your eyes closed or looking elsewhere.
You can feel the folds with your nails, but you can't see them.
Grooming depends on sensations coming from the skin to direct and steer your actions.
If you use sight, it is so powerful that you rely on it immediately and all your technique changes.
The situation is further complicated, for all the other primates, by their thick coat of fur.