In high school, at least from what I've seen, mathematics courses never use Greek lettering in their description of concepts, with the notable exceptions of $\Sigma$ for summations, $\Delta$ for changes over time, $\pi$ as $3.14159\ldots$, $\tau$ in physics courses, and $\theta$ for basic sines, cosines, and tangents. This behavior is mirrored in typical college placement exams, such as the SAT or AP exams, which also do not typically use any Greek lettering.
Yet, when students enter college, classes and instructors do use Greek lettering, and use it without preamble; they assume students are familiar with such notation. Yet, typical freshman are not familiar with Greek lettering, and are not sure how to draw, pronounce, or think in terms of, such letters.
Is there a specific reason Greek lettering is deferred to the high school $\to$ university transition, and, more generally, for Greek lettering in the first place?