Because *sere* was never a Latin word in this context. Instead it was
Etymology: OE. séar corresponds to MLG. sôr (LG. soor), Dutch zoor:-OTeut. *sauzo-:-Indogermanic *sausó-, whence Lith. saũsas, OSl. suχŭ, Gr. αὑ̆ος dry, Skr. çōsha drying up, withering. OE. seems to have had also a synonymous derivative síere (:-*sauzjo-), which in later Eng. would be represented by the same form as séar.