Dr. Dwayne
Self proclaimed tagline king.
I assume because other countries where you can buy firearms do a much better job of background checking people.
That, and most probably realise that there is literally no need to let random civilians carry powerful assault rifles besides financial reasons.
Yes, background checks, licencing, waiting periods, etc all help.
But definitely the biggest difference is that carrying a firearm in public is considered normal in the US. In most other countries carrying a gun in an urban area will result in a significant response from citizens and police.