Aiming at a person is way different than shooting loads of bullets into a densely packed crowd. Reflexively shooting with a rifle isn’t as easy to shoot accurately as some think that it is. The real immediate issue with gun control is home defense. If someone is breaking into your house, you have far less time to personally fight back against the intruder than it takes for the cops to get to your home.
I also forgot to mention, shooting up from the ground is also difficult as well, the shooter had hotel rooms and ceilings shielding him.