I guess whatever the OP decides constitutes an "assault rifle" it's the same answer:
Because the 2nd amendment says I can.
Are you in a well regulated militia? Who is regulating you?
Actually the 2nd Amendnent does not say you can. Over the decades Congresses and judges have rendered interpretations that say you can.
They have also defined the point where you can't own wespons designed for use in combat.
So your precious idealized favorite war toy is on a fine line and in a precariously unsafe position if society, morally and politically decides, as a whole, to adjust the ban on the sale and possession of war toys to include the ones that were banned before and could easily be banned again.
You are not in a well regulated militia are you.
The 2nd Amendment says nothing about you then.
Better start organizing but remember .... regulated by whom?
You might end up more regulated than you are right now by citing the
Assault rifles are short, compact, select-fire (i.e. both semi- automatic and full-automatic) weapons that fire a cartridge intermediate in power between submachinegun and rifle cartridges.
Why are you banned from owning and stockpiling RPG's for example?
You must believe the 2nd Amendment says you can. Why don't you have that right? Why have you stopped at a right to possess certain types of assault rifles?