If people on benefits work "cash in hand" why are the employers not prosecuted?

They prosecute the benefit claimant for defrauding the system and I find it hard to believe that the employer is not aware the person is on benefits so he/she is aiding and abetting the fraud. This is besides failing to pay NI contributions and tax. Seems unfair that the benefit claimant takes all the blame. What are your views please?. Btw I am not on benefits.