There was a staff discussion about this when you brought it up before, and we opted to keep things as they are.
Now that you've been here for a while, I'm sure you've noticed how friendly the community is, and how much everyone likes to interact with each other, especially on projects and builds they are working on. The idea of build threads, is to show the progression of whatever you're working on over time. As you post up, people are going to comment, ask questions, give feedback, etc. If you are going back and retroactively changing posts in your thread, the rest of the thread will become disjointed the longer it gets.
Hopefully this explanation makes sense.