I’m not sure which meaning of ‘should be able to be voided’ you’re using. Do you mean ‘Why do think it’s legal to void it’ or ‘Why do you think it’s legitimate to be able to void it’?
In the first case: My employment contract does exactly this. It’s become kind of a default clause in contracts. Researching this you’ll find a lot of websites (in German) that say that the clause is ‘abdingbar’ (which I translated as ‘voidable’).
In the second case: I didn’t say I thought it legitimate, and I don’t have a strong opinion on this.