Because the amount of control you would need to have is more complex than you think.
You can’t just do a thing. You’ll be labeled a domestic terrorist. You’ll be taken away and thrown in a black hole somewhere, assuming you’re not killed doing whatever it is you want others to do.
You need organization. Order. Leadership. Clear and effective objectives to make progress towards a defensive and well resourced enemy, namely the government I presume.
Because you would need to commit to war. And to be able to war, you would also need to garner the support, resources, numbers of variably skilled individuals, and logistics to wage your war.
Allowed is not the word. Tolerated is more accurate. And what was the 2A written for? To wage war whenever you don’t agree with something? That’s what the opposing side with feel, say and think. Whoever you think is the enemy also thinks they’re the good guys.