Root versus leaf, and everything follows
Maca is a root vegetable — closer, botanically, to a radish or a turnip than to anything in the tea aisle. Roots store energy as starch, which is why maca powder is heavy, beige and faintly sweet, and why it behaves like a flour in liquid rather than like a tea. Matcha is a leaf, and leaves store amino acids and chlorophyll rather than starch, which is why matcha is light, green and drinkable on its own. Nearly every practical difference between the two comes back to this one fact.

