The AR-15 is just popular because it’s the honda civic of rifles and people fetishize it for the same reason highschool car guys fetishize honda civics. It’s cheap, ubiquitous, and you can modify it to do basically anything you want. You can get an AR-15 chambered in anything from a .17 varmint round up to .50 Beowulf if you really want. You can slap basically any attachment or replacement part you can think of on it and it will just work. It’s also fairly robust so maintenance isn’t a huge concern. It’s just a generic, cheap, and highly versitile platform. As far as “big bad guns” go the AR-15 isn’t anywhere close to the top of that list. To anyone who actually knows about guns the AR-15 is basically the great value brand rifle. It’s what you buy when you don’t know what you actually want.
You also have a bit of the glock phenomenon going on. It became a thing a while back where a lot of people just started calling all handguns glocks even if they weren’t actually Glock handguns. Similarly now you have people calling any black rifle with a handguard and a picatinny rail an AR-15 even if it’s not even close to an AR-15.