Employment contracts are not like "any other contract". Most European countries will have specific legislation covering workers' rights, which will typically supersede contractual clauses in case of contention.
That being said, the role is in Germany, where it is, to a limited degree, possible to sue for damages resulting from not honoring the notice period. It is also common to add an additional limited a priori damage value into the contract. Not legal advice. [1]
Anyhow, for anything below a managerial role I've never seen a company enforce those rules. They're mostly a deterrence, I suppose.