Because in the 21st century we no longer care about the Rule of Law?
Nice try, but no. Because in the 21st century, an "on-demand" army or a volunteer militia doesn't work. In fact, it hasn't worked since the 19th century. Wars now involve much greater degrees of technology, distance and speed. Therefore if you have the remotest plans to be in a war, whether defensive or offensive, you must have a standing, well-trained army, established command, technological parity with or superiority over your possible opponents, and stockpiles of weapons. In most cases you also need nuclear weapons.
I'm not arguing with you how the change to the Constitution must be implemented, whether by silently ignoring it or trying an amendment each time (a very laborious and politically difficult task -- one of the reasons the US constitution is so brittle). But the fact is, if you took the current text and all its amendments at face value, you could not run a modern country -- you could run something at the level of Somalia. Times change and circumstances change.
If we can't muster the political will for an amendment, well, perhaps we should consider what that means.
It means that it was ill-designed. I don't know how long the US will survive with the constitution at its nominal core, but the "Constitution" is now only a small part of and often in contradiction with its real constitution, i.e. the body of written law, common law and political precedent that really determine how it is run. The fact that this is so is really a reflection on the fact that it was designed to be too brittle. This was intended as a precaution against its modification, but of course paper cannot stand against human will. If the people want a liberal system, they will keep one; if they don't, they won't, and the authority of the Constitution is merely a fiction, as powerful as the amount of belief in it. Today, belief in some parts of it is strong because it agrees with what we think should be right -- freedom of association, for instance. Belief in other parts of it is nonexistent -- the parts about a standing army, the separation of church and state. Free press/speech is in the middle -- some people believe in it and some don't -- and that's why the state of free speech and press is pretty murky in this country today.
I don't mean to cruelly burst any bubbles, but you know perfectly well that that's reality -- words on paper are only as strong as the amount of belief in them.