Except you can modify semi-auto guns into fully-auto guns.
(And no, semi-auto means you cannot simply hold down the trigger to fire a stream of bullets; you have to pull it repeatedly, and my understanding (as a non-gun person) is that "assault weapon" refers to a combination of features that makes guns look scarier / more military-like)
Semi-automatic rifles fire a single bullet every time you pull the trigger, the reason why they are semi-automatic is because when the trigger is pulled the bullet is automatically cocked, fired, spent casing ejected and the next bullet loaded. This saves on a great deal of time as no manual bullet loading is required. A fully automatic rifle does all that and brttttts automatically for as long as the trigger is held
Contrast with one of the more common rifles in the Murrican market like the nugget, where this is all done manually with the bolt action, or any pump action shotgun which does the same with the pump action
"Assault weapon" is a pretty memey term by the media same way a police officer with a shotgun became a police sniper in Ukraine
It has a non-memy definition of a fully automatic rifle that has selective fire (firing in bursts or full retard mode) with a detachable magazine
They don't look military-like, it's more that most world militaries really like them thus making them iconic of military. They are bread and butter of most world militaries, militias and militants, like fabled AK-47 which all media across the world can recognize as assault weapon of AK-47 style assaulting. You can also just paint all guns black and slap tacticool stuff on it like in COD but there's not much point in removing any character from such machines
Oh! Almost forgot. Assault rifles by definition are capable of burst and full brrttttt modes, but I don't know US gun law but most governments are scared by their citizens being equally as armed as their army so in countries where assault rifles are legal they sometimes do stuff like placing legal conditions for their sale, like the elimination of full auto mode or the capacity to fire high-performance sexy ammunition
If this is the case in Murrica then it seems what people are trying to stop (if they knew what they were talking about) is someone buying a civilian certified assault rifle (with no full auto mode or sexy ammunition) and modifying it to be on par with the military one all over again