Have you ever thought why? Because in Islam, you cannot award someone Capital Punishment unless there is absolute doubtless proof. Even the concept of four witnesses cannot work in todays age like it used to back in that time; let me give you context, back in the time, even when four witnesses were drawn, it was made sure that they were absolutely pious, known to be truthful and trustworthy, and had NO past accounts of Islamic law or general society discipline breaking. That was the standard. Therefore, mere words or four unknown witnesses are not enough to appropriate Capital Punishment as an award to someone.