I don't mean this to be incendiary, but, do you think perhaps this reflects on your recruitment pipeline, the standards you hold interviewees during the discussions about their experience, and the way you review resumes?
There is of course a possibility that someone has rehearsed their interview happy path down to the word and they can easily describe complex technical concepts without understanding them because they're quoting a script, but an interview that can be gamed in such a way may highlight a problem with the interview.
If most of your interviewees are failing to write basic code, the issue probably isn't the applicants.