It's really just preference, I guess. The break of flow is the big thing for me, but I think I'd have trouble even in a game where everyone is using first. One thing that was mentioned above is that having the character(s) named in a post helps me remember them specifically against the other characters. Especially in big games, it's hard to keep track of everyone as individuals until you know them and, for me, having the cue of their name mentioned helps. I think that first person is lovely when it's used for a narrative focusing on one person's point of view, such as Nick in the Great Gatsby or Katniss in the Hunger games. These stories are anchored in that specific person's point of view but by an RPGs very nature you have more than one character, often several so I feel like third person moves people away from "focus on my character" to more "focus on my character's part in the greater story". RPGs are a group activity after all.
I also find the repeated use of the word "I" to be jarring however, there are people who write in third who start every single sentence with He/She or their characters name so this is more about lack of experience than tense.
tl;dr I'm basically just old and grumpy.