In German law that would, as a default, actually make the whole contract void, yes. Many contracts therefore contain a clause that hedges against that situation (i.e. something like "if one or more of these items is found void, the rest still applies")
Specifically, the idea of a voided clause voiding the entire contract is sometimes a feature -- severability is not always desired. A simplistic example would be a contract with two clauses, one specifying that A sends B widgets, and the other specifying that B sends A money. You would not want those to clauses to be severable.
