I once asked the same question. I haven't read the whole thread and i'm sure somebody will have already said this but what the hell, i feel like typing.

A lot of ancient artworks show Greek hoplites holding their spears over-arm. I tried it one day with a mop handle, hitting the sofa. Under-arm, you can get a good hard thrust. Over-arm, not so good, so i came here and asked the question you asked.

It's so simple when you think about it though. The ancient Greeks used very large round shields and fought in tight formation, shield-wall, side by side, presenting a wall of shields.

If you hold the shield under-arm you have two big problems. Firstly the guy on the right's shield is going to restrict you from doing anything but a straight-forward thrust. Secondly all you're going to hit with a straight-forward thrust is your enemies shield, over and over and over again.

However! Overarm, your spear is above the man on the right's shield, giving you more room to move it about. Not much more i'll grant you that but more room nonetheless. Also, you can poke over your enemies shield rather than directly into it, and rape your enemies face with your spear.