Actually lots of people do and in terms of practicality they have a case.
Munitions manufactures don't want it because they know with big magazines, yahoos will burn through more rounds and it makes them money. Fact is though that if the US implemented a policy on magazine capacity, manufacturers will follow suit or lose access to the world's largest market.
You are right though, magazine limits alone aren't enough. As I've said repeatedly and no one has ever addressed, the ATF states that the vast majority of illegal guns used in crimes come from a small number of gun dealers that contravene the rules but legislations leave them little ability to investigate or prosecute.
No, they don't. You just think they do and keep constantly repeating it to try and convince others and yourself. Only 8 states have magazine capacity limits, and one of those states, Hawaii, only has limits on handguns, so 41 out of the 48 states have no limits on rifles. As stated earlier, one of those states, Colorado, doesn't even enforce the law because of the lack of cooperation from their police forces.
https://www.usatoday.com/story/news/nation/2014/01/30/colorado-gun-laws-enforcement/5055523/
6 out of 48 states is not "lots of people."
The U.S. will not be implementing national magazine limits, not when states like Wyoming are willing to give Magpul an $8.3 million interest free loan to relocate from Colorado. It would not be a stretch to say it could lead to civil war if they tried to nationally limit magazine capacities.
You're also wrong about ammunition manufacturers. Ammunition manufacturers never comment on mag capacity. Again, you know very little about the industry you are trying to criticize. The vast majority of people do not have hoards of ammunition they are constantly blasting off. The average gun has less than 500 rounds through it, and spends the vast majority of time in a safe. That's one of the reasons people are such lousy shots, including the police when they are in a high pressure situation. The vast majority of gun owners are just that, owners, not shooters. Guns are cheap. Ammunition is expensive. 5.56, the ammo used by the Vegas shooter averages 50 cents a round Cdn for a generic round. You go the range and shoot off 140 rounds, or 5 mags, that's $70. Ten trips like that and you've blown away enough ammo to buy another rifle.
Hunters only have a couple of boxes of ammo because that type of ammunition is expensive, typically at least a $1 Cdn a round. Precision ammo can be closer to $2 or more per round. The people who blast through large quantities of ammo, like IPSC shooters, reload, which is a small portion of an ammunition manufacturers business. If you actually walked into an outdoor store, you would see that.