The Greek roots that appears in English words aren't really foreign, they're part of the English language and have been since forever.

That may be the case, but most people don't know what they are and what their original meanings are, so they might as well be foreign words.

So they haven't heard of epiphenomena, epithelium, epigenetics, epigraphs, epiderm, epithets? Or monosyllabic words, monoski, monochromes, monogamy, monograms or monologues. It's not hard to guess the meaning of the roots.

The only word of these that I would assume any given english speaker knows is monogamy, and maybe monologue. I myself know those, epithet and monogram. Perhaps I could guess on the others, but I certainly wouldn't know what epi- meant.