Because people are generally lazy by nature. This overlaps into both the criminal and non criminal element for the most part.
It's similar to asking how come drug dealers really don't know tha purity of the drugs they injust or sell. People want these things done for them, they have little initiative to read or study how something works and go and make a part or construct a simple testing aparatus to test the drugs they sell.
They prefer to have someone else tell or show them what to do.
If made more difficult, they probablly would not know which way the bullets go in the magazine. They have no interest in how something works, only that it does.
This is good in a way because it limits what they can destroy. If they wanted to actually work, they probablly would not be shooting and selling drugs to people. The lazyness carries over to every aspect of their exhistance.
Having grown up in some bad places, I can tellyou that many have no social security number drivers license, and have never paid taxes. This while owning a half dozen homes in other peoples names, and vechicles from rent a car agencies under other peoples credit cards.
Criminals are looking for the easy way to make a lot of money, if it requires work, they will pay someone to do it or buy their way in or out of it. You can sell a bad guy a m14 for 5000 dollars easily. if he wants it and you have it, he will pay for it, money means nothing to guys who make ten to twenty thousand dollars a week.
If they want a machine gun, they will ask around and pay whatever it cost, "if they are on that level" otherwise they will deal with whatever they can get the easiest. Very few know the difference nor care. Most never shot the guns they have, they have people who do that for them.That's where Mercs and ex special forces guys "gone bad" make a lot of money. They can get things done for these guys for a very large amount of money. They always hire out for stuff that really needs an expert. Most that I met don't know crap about weapons, they are lucky if their gun even fires.
Guys like this crazy person who killed these poor innocent people are a rarity, he just bought what was available to him, and having a fair amount of intelligence, probablly practiced out in the woods some place, until he reached an acceptable level of marksmanship. In his eyes.
I am sure he did not fully know how many people he would or could actually be able to kill. He just had a plan and followed it.
He had a pretty good knowledge of how people react in this type of situaion, "probablly did his research" and was very analytical in his execution.
I would have thought someone would have resisted, but I should know better. If he was able to get a machine gun without drawing attention to himself, he may have tried. But he stayed under the radar juat enough to accomplish what he set out to do. Which pisses me off to no end.