Any of the drones used in the Ukraine right now would be able to do a lot of damage. And those things are relatively low tech.

The reason these horror scenarios aren't so common is that most people simply are not suicidal murderers. It's always been easy to target crowds of people. But it's not that common for people to actually do that. When it does happen, there usually isn't a happy end for anyone involved with the attack.

Any determined idiot armed with a knife, baseball bat, truck, improvised explosives, etc. would be able to do quite a bit of damage. And in the US you can of course add all the light military gear that lots of people there buy for fun (assault rifles, etc.). That sometimes goes horribly wrong of course but you don't need drones to do a lot of damage.

Semiautomatic rifles were legal in the US for a time before someone shot up the first high school. But now it's a concept in the public consciousness and schools need to have shooter drills due the frequency.