On a channel with 20 people that would mean one screenful of hellos. It makes sense not to greet everyone, in order not to be rude to the rest of the channel. If you must say hello to somebody you know, do it with a private message. The same applies to goodbyes. Also note that using your client's facilities to automatically say hello or goodbye to people is extremely poor etiquette.
Nobody wants to receive autogreets.
They are not only obviously automatic, but while you may think you are being polite, you are actually conveying yourself as insincere. If somebody wants to be autogreeted when they join a channel, they will autogreet themselves. Remember, people on IRC form their opinions about you only by your actions, writings and comments, so think before you type. If you use offensive words, you'll be frowned upon. Harassment is defined as behavior towards another user with the purpose of annoying them.