I think early bird is best reserved for close friends of the host. I invite close friends, somewhat-close friends, and maybe tangential friends (that neighbor who seems nice but I don't really know well) to parties, and if it's the last group who shows up first conversation flows less smoothly.

Also if you're first, be ready to help set up!

Are you thinking only of parties with friends? Arriving early can be an awkward and intense social situation; it's just you and the hosts.

Provided I know the host well I consider those early arrival situations much easier to handle than arriving late when the gathering is already in full swing. I know it's absurd, but it almost feels like interrupting.

I prefer too ease into these events by starting early and with a small group instead of making an entrance to a full room.

"Why don't you want to be the first person?"

In some cultures appearing a busy person can be important. Showing early to a party could send the message you had nothing to do before. Not that i like or do that, but some people could pay attention to such details.

It has nothing to do with looking busy. It's simply awkward when you go to a party and you're the only guest present and you don't know the hosts very well.

Even more so if the hosts are still frantically finishing preparations. As an acquaintance, do you offer help? Go for a walk around the block? Best to just avoid that altogether by showing up a little “late”.