Why would the listeners of so called wise men would call their brothers fool

In reference of merchant of venice ac1 scene1

Gratiano’s sarcastic comments about Antonio’s sad state - He said that they were people who try to cover their faces intentionally with seriousness so that others would think them as persons of wisdom gravity and profound conceit. Such people also make others think that whatever they say should be taken as God’s voice and when they speak, no one else should speak. He became more sarcastic by adding that such people remain wise till the time they do not open their mouths. The moment  they open their mouths, the listener would call them fools and thus get cursed.