Actually mass shootings like these happen all the time, most often in cities that already have strict gun control laws (looking at you Chicago), it's just that those are usually black-on-black and gang related, so the media doesn't bother to report on them. It's only when they need to frighten the public about guns that they start looking to cherry pick examples that they hope will support whatever narrative they're looking for. Even if there wasn't a big push to strip gun rights going to the Senate soon this sort of shit would still be happening, you just wouldn't be hearing about it.

The thing is that going by overall population the violence here in the states isn't that much worse than anywhere else. Yeah gun violence tends to be up, but that's primarily because that's the weapon of choice here, in countries that don't have readily available guns people just stab each other. Also worth keeping in mind that the top-five cities in the US for gun related violence are all major democrat led cities with strict gun control laws. And we already know from many surveys being done that most criminals in federal prison wouldn't have done the crime that got them there if they knew the victim was armed or likely to be armed.