I can’t say for certain this is true for everyone, but: because we care too much.
#Because we care too much about the forum
This is probably the most common cause, and also results in the nastiest arguments. The staff have taken a laissez faire approach to moderation on the forums – members can misbehave with 0 consequence. As testament to this, when someone was harassing others via PMs on Kunena, ROBLOX enabled PM blocking instead of dealing with the problem user. It’s not uncommon to see someone have every thread they post on locked or every post they make erupt in argument, and roam free for months before any action is taken, if that even ever happens.
This is extremely bothering to anyone who cares about the forums. When someone acts up in that manner, regular users (and even community leaders) are powerless against them. If efforts to get them to stop have failed, another user may try to bring their behavior to light in vain hope that the staff notice and take action – not necessarily removal, but enough to accomplish what asking nicely couldn’t. In hindsight, it might be obvious that this is one of those “when you gaze into the abyss, the abyss gazes into you” instances, but when you care (too much) about the forum, you feel obligated to do at least something.
#Because we care too much about ROBLOX
I suppose this is the case with any software producer, but collectively the community doesn’t trust ROBLOX. This is really evident in any controversial or web-related update thread. When we feel as if ROBLOX isn’t being attentive, we may be less inclined to constructively and slowly come to an agreement than mob-spam the thread so it’s impossible for ROBLOX to miss it or be confused with what the community wants.
#Because we care too much about individuals
Someone may have a misunderstanding – they could be ignored, but another user might leave with a guilty conscience if they left them hanging. As a hypothetical example, OP wonders why they can’t get a poorly-designed game popular and PersonB tries to explain how they can improve it. OP is obstinate and defends all of the flaws of their game, PersonB gets frustrated over time without noticing it, and eventually a conversation aimed at helping OP has devolved into a dumb argument.