Old fashioned email etiquette and "netiquette" favored brevity and omission of pleasantries (and trimming quoted messages, etc.) because people were reading on 300 baud dial-up and it made the difference between taking 5 seconds to load your message or maybe a minute or more.

It's largely irrelevant for any technical reason today, but the old greybeards still cling to it (I'm one of them).