The "english-literate" world (emphasis on literate) is, or historically has been, very familiar with Greek glyphs.
And that's just for humanities.
The mathematic and physics -literate world, doubly so. Everybody uses pi, theta, sigma (e.g. the summation formula) etc symbols...
I would be shocked if even 1% of native English speakers could list the full Greek alphabet much less have a comprehensive familiarity with what each letter means across all branches of mathematics. I would be shocked if even 5% of native English speakers could tell you what theta generally means in geometry, much less all its other meanings.
Many of these characters are hard to type on regular keyboards, which is probably what the parent commenter was referring to.
