Why do Catholics consider praying to Buddha to be idolatry and praying to Mary is not?

Mary and Buddha are both human and lived on earth (Mary 20BC and (Buddha 563 BCE). But why is it praying to Mary's statue called veneration and praying to Buddha an idolatry? Why Shadrach, Meshach and Abednego refused to venerate the statue of Nebuchadnezzar, King of Babylon. Did apostles and early 33AD Christians pray to early prophets like Moses, Elijah? Just like the Catholics did to statues of Mary and apostles today?