For example, here is the definition of zealous I find when searching
Why are hard-core, overkeen and literary in grey?
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It turns out that the definitions you get from Google actually the definitions of Oxford dictionary. So it's actually because Oxford dictionary doesn't give Google the link to those word:
But I don't know why it doesn't link. Apparently those unlinked words exist in Oxford. And I don't know why Google can detect some words such as devoted or vigorous to link it, too.