Non Buddhist can say Hello to Buddha or Monk?
Then the monks, delighting in &amp; approving of the Blessed One's words, rose from their seats and — bowing down to the Blessed One and circumambulating him, keeping him to their right — went to Ven. Sariputta. On arrival, they exchanged courteous greetings with him. After an exchange of friendly greetings &amp; courtesies, they sat to one side.
https://www.accesstoinsight.org/tipitaka/sn/sn22/sn22.002.than.html
Perhaps it was a very old tradition to respect the elders and worthy people or even people with power.