How about because we do not want to end up like Australia?
Wild pigs in the US- 4,000,000
http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5313597.pdf
Wild pigs in Australia- 23,000,000
http://www.telegraph.co.uk/earth/wildlife/3308375/Australia-has-more-wild-pigs-than-humans.html
Wild pigs already do billions of dollars in damage every year in the US and we hunters are the only thing that actually prevents them from taking over like they have in Australia (and that is not the only species that is breeding out of control in Australia). In addition to that is just general pest control.
Or how about because we still have dangerous (not just annoying) animals in the US like wild bear- http://www.seattlepi.com/news/article/Alaska-woman-recounts-terrifying-ordeal-with-bear-3707497.php#ixzz20eC0iwhF
Or how about because we feel the need to protect ourselves? A friend of mine lives on the border, you can literally see the border from his home (or at least the place where the sign that is supposed to mark the border is). Drug runners regularly use his property to smuggle drugs in. If he called the police it would take them 20 minutes or better to get to him. Do you think it is a good idea for him to be disarmed? And before anyone says it, he cannot move, his grandparents bought the property, he cannot afford to purchase a new home, and no one in their right minds would buy that property. Then you have a friend of mine who was raped. She carries a gun on her because she doesn?t want it to happen again. And of course you have incidents like this- http://www.msnbc.msn.com/id/31416285/ns/us_news-crime_and_courts/
There are other things but let us focus on the economics for a moment.
The basic fact is that attacking the gun industry is harmful to the world?s economy. You may not realize this but your police force and military practices with ammunition. That ammunition is cheap because the US produces a tons of it and exports. Either practice goes down or costs go up.
Plus, you are talking about tens of thousands if not hundreds of thousands of jobs. You have gun shop owners, ammunition manufacturers (both large companies and small businesses), unrefined resource providers, military contractors, etc. Where do you think the materials to make these firearms and ammunition come from? What about all of the leather used in holsters? Where do you think the computer chips used in gun owners tech comes from? And on.
Your attack on the gun industry would send shock waves throughout the entire economy. There is no major part of the US and world economy that is not somehow connected to the firearms industry. How many ranchers are going to lose money when the demand for leather goes down? How many businesses built around firearms companies are going to survive if the largest business in the area goes under? How many mining companies are going to feel the effects of a fall in the brass market? How many ranges are going to go under and thereby force Police Departments to build actual ranges and how much money will that cost? Etc.
Then, in addition to that, you are going to have to get rid of the firearms somehow. Ignoring the how for a moment, let us instead focus on the cost to do that. First you are going to destroy hundreds of multimillion dollar businesses directly unless you pay them off (lots of money there). Then you have the 80 million gun owners in the US. If ever gun owner owns $300 in guns and firearms accessories that equals $24,000,000,000 you have to pay them (unless you are just going to take the guns and say fuck you to every gun owner). Of course that number is vastly underestimated. I myself own several thousand dollars in firearms and accessories and I am rather young. Plus, most bolt action rifles cost over $300, most semis cost over $600, and most pistols cost over $400.
In other words, firearms are necessary in the US and it would cause a huge harm to get rid of them. Not to mention the basic fact that it is impossible to get rid of them. 300,000,000 guns do not just disappear because you want them to (not to mention all of the guns outside of the US and all of the illegal guns).
BTW guns are not illegal in the UK, just heavily regulated.