Why are the small brown structures on a strawberry called achenes?

The small brown structures on the surface of a strawberry are called achenes because they are, botanically speaking, the actual fruits of the plant.12

An achene is defined as a small, dry, indehiscent (it does not split open at maturity) fruit that contains a single seed. In this structure, the seed remains free within the fruit cavity, attached to the ovary wall at only one point.312

While a strawberry is popularly considered a fruit, it is technically an accessory fruit because the fleshy, edible part is actually a much-swollen receptacle—the thickened tip of the flower stalk—rather than the ripened ovary itself. The true fruits are the tiny, brown, seed-like achenes embedded on the surface of this receptacle.12