I was asked the question in the title some time ago. I answered that I really don't know (I suggested a possible explanation which turned out to be false). The student suggested a YouTube video by prof. Merrifield:
<https://www.youtube.com/watch?v=CiHN0ZWE5bk>
I found the explanations quite convincing. If you care to watch the videos (they are not long), I'd like to hear your take on the question.
Best wishes,
Antti Savinainen, PhD
Finland
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