The measure of true celebrity is you always learn something you never knew before. The one thing I learned from ITV's documentary Our Queen was that it's not her habit to say either hello or goodbye when meeting people.
It makes sense. People are always lined up waiting for her and so it would clearly be ridiculous to expect her to explain herself. Similarly when she withdraws she can't say "well, it's been wonderful but one must dash" or any similarly pat line.
It also saves time and breath.