ANSWER: Because he levitated frequently!
His levitation was witnessed by hundreds of people, including Pope Urban VIII and two cardinals.
St. Joseph of Cupertino ((1603-1663 A.D.) was often observed levitating in the air during deep prayer. Once, he levitated to the top of St. Peterís Cathedral for two straight hours.
Levitation was also experienced by saints such as: St. Teresa of Avila, St. Philip of Neri, St. Ignatius of Loyola, St. Catherine of Siena, and St. Padre Pio. On one occasion, St. Teresa of Avila and St. John of the Cross together levitated to the spires of St. Peterís Cathedral. On another occasion, Sister Anne of the Incarnation testified that St. Teresa of Avila levitated one and a half feet above the ground for half an hour.