The clothing radical 衤 dresses the character; 库 supplies the sound. Early Chinese 裤 were leg tubes without a seat — trousers proper arrived with northern horsemen, because you cannot ride in a robe. Fashion followed cavalry.
King Wuling of Zhao's 胡服骑射 reform (307 BC) — 'wear barbarian trousers, shoot from horseback' — is the classic case study of pragmatism beating tradition: he put his court in pants and his kingdom in the saddle.
The clothing 衤 warehouse 库 for your legs — TROUSERS.
Trousers count with 条 (strip) not 件: 一条裤子 — Chinese measure words see pants as one long strip, not an item.