Why is the standard of refereeing so bad in the Premiership?

Let us be honest here the big clubs Manchester United are always given penalties , and the whole game goes in favour of Manchester United. Even in the match between Manchester and Totten ham Hotspur, Harry Redknapp had strong words to say about the referee Howard webb, he has been a culprit on repeated times. Another one is Mike Rily. They are just terrible .