Since I am not a robot, I enjoy being greeted and thanked, as is the custom in human societies. It also provides a tiny opportunity to express myself, which I relish. Chastising people for using common courtesy is when you know your website (and personality) has jumped the shark.
The effort to "purify" StackOverflowStack Overflow will eventually lead to its undoing. Imagine how easy it will be to be a nuisance when all you have to do to make trouble is say "Hello"!
EDIT: I'd also like to add that this will create chaos, as every single new user will ask every single time why their salutation/greetings keep getting deleted. Then they'll be flooded with links to this question by the nerd patrol. A full 5-10% of all SOStack Overflow content will be about the salutations/greetings. Why? Because it is completely normal and natural in human communication to use these constructs, so it's intuitive that they should appear when you type them.