Why do sabbath keepers believe they have to keep the conditions of a covenant they are not a party to?

Cherry-pick. Cherry-pick. Cherry-pick. Many christians do love to do this. They ignore the inconvenient (to them) parts of the OT, then lean heavy on the ones that support their pet bigotries or that they can "practice" to make themselves feel/look virtuous. The common term for this is "hypocrites". Check the NT on "hypocrites". Jesus hates 'em.