"Imo, colleges are better at both recognizing a lack of interest and a not great fit "
WashU btw did not have a supplemental essay until recently so there really was no way for them to figure out who was really interested since they didn’t have a why us and was going off the common app essay. They did this to get more applicants of course.
Now they added the essay because they wanted to improve their yield. They also consider demonstrated interest so a waitlist is actually not that bad if the OP in fact did not visit or show any interest.