It's also done to prevent sneaky edit wars that would otherwise go unnoticed. Not everyone is PVP centric. A certain limited amount of editing is okay (eg, fixing mistakes) but it shouldn't be used as a means of responding to other actions. New posts for that.
((I've been doing this for the last eight years, and that is a pretty rare event. Metagaming based on actions is a problem, sure, but the proper solution isn't banning edits; the proper solution is private messages. If you just require new posts, that still allows metagaming, in that whoever posts most recently will have the most up-to-date counter. This advantages people with a lot of free time, or few games, and can therefore check the thread often. In practice, this is very unusual, since forum games almost always lack the complexity for counter-counter-counters to exist, and those that do typically have actions through PM... but it's the situation you're worried about.
Additionally, banning edits is simply impractical unless the GM has email notifications for every post in the thread, or
very actively checks it. If I want to metagame and edit my action to respond to someone else, I could just... change the action, and not say I changed it. The only thing that will inform the GM I changed anything is the little "this post was edited" message, which could just as easily be spellchecking.))
Hook the Unseen Sword to my belt, as is the style
"Huh, it's not there... it was there a moment ago? Weird."