Because its fruit is in the ground.
Almost all plants on land bloom and bear fruit on the ground, but peanuts blossom and bear fruit on the ground, so people call them groundnuts.
after peanut seedlings were unearthed, they began to blossom after 18-25 days. In the evening, the yellow flowers slowly appeared, and at about 7 o'clock the next morning, the flowers opened and withered that day. On the 4th day after flowering, its ovary stalk stretched and grew under the soil. After about 5 days, the fruit matured.
Arachis hypogaea Linn.: panicum miliaceum of Arachis hypogaea, Leguminosae. The roots are rich in nodules; Stems and branches are angular, and leaves are papery opposite; Petiole stalks at the base, oval to obovate, obtuse at the apex, hairy on both sides, with eyelashes at the edge; The vein edges are connected with each other to form a network; The flower is about 8 mm long; Bracts lanceolate; Corolla is yellow or golden yellow, flag petals spread, wing petals are separated from keel petals, oblong or oblique oval, style extends beyond the pharynx of calyx tube, pods swell, pods are thick, and flower and fruit period is from June to August.
resources peanuts _ Baidu encyclopedia