Why doesn't English have a word that means both Hello and Goodbye?
Probably, good day is the English equivalent, although a bit old fashioned.
used as a greeting or when saying goodbye during the day
[Cambridge Online]
English - Ciao!
ciao interj.
Used to express greeting or farewell.
[American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2011]
English continues to borrow words from said languages.