I don't think we forget that part, it's just irrelevant in our opinion. If it's acknowledged that the amendment was added so the people could fight the government should it no longer serve them for whatever reason, then the government can't be allowed to determine in any way what constitutes a militia. Otherwise the government could define militia in a manner that negates the entire amendment and it's purpose. Thus only the people outside the normal government processes can determine what a militia, which opens it up to multiple valid interpretations and implementations.
Of course this idea requires you to believe the founders added irrelevant text to constitution, which doesn't seem likely given the thought put into the wording.