Dirty eating utensils like cups and bowls are a health hazard. The Pharisees were very particular to wash them in a ceremonial way which enabled everybody to see that the outside was clean. It was like that with the Pharisees themselves and so Jesus used their religious habits as a parable. He knew the inside of their hearts and saw through their holy rituals to the inner corruption that motivated their lives.
 
Although they were supposed to be leaders of God’s sheep, they were like the corrupt shepherds of Ezekiel 34:1-4. Their interest was not in being trustees of God's flock but in advancing their own selfish agendas. Instead of wanting more honour to go to God, they really wanted glory to come to them. Their objective was to be seen as being spiritually important and honoured by the people and to get whatever they could to make themselves rich.
 
They could not see what they were like. If man is blind and tries to fly an aeroplane, he is a danger to himself, the passengers and the people on the ground when he inevitably crashes. The Pharisees could not teach God's law properly or take people into truth because they neither knew God nor did they practise living for Him themselves. Jesus called it 'hypocrisy' which is 'play-acting'. He rebuked them, warning them of judgement; and because they were living spiritually deadly lives, no wise person should follow them.
 
Hypocrisy is easy: make up your own script which includes religion and then live a life which gives you whatever you want. Religious activity and ritual can easily mask a corrupt heart in the eyes of other people - but not God. Today might be a good opportunity to test your own motives and see if you are as clean inside as you appear outside. If not, the remedy is not to do more play acting, but to come to God in repentance and ask Him to change you from the inside. When your deep desire is for God to be glorified, then your actions will have integrity before God. Anything else is acting out your own playscript: woe to all who do, but blessing to those who repent.