For example, here is the definition of zealous I find when searching
Why are hard-core, overkeen and literary in grey?
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.