Why do you need to post about other people's goodbye posts? Tired of seeing them? Nobody is forcing you read their posts, or comment or them, or anything else. Most of the posts on the forums are some kind of rant or complaint. You'd just be replacing one kind with another.
Some people have played for 6+ years, leaving is a big deal for them. Some people have put thousands of hours into the game, community, etc. Its easy to understand the reason or emotion behind it.
Even if they understand nobody will notice or care, they owe it to themselves to explain to the community, especially when they feel they are more being forced out rather than "moving on" or whatever.