The ATF still allows you to "self manufacture", as long as it is a legal to own firearm (IE you can't build machine guns) and legal to own said item in your location as per state or municipal law. I believe you do have to serial number the item, and you may not transfer it to another person unless you have a license to manufacture firearms for sale from the ATF. You can get the directions for machining an AR-15 lower receiver forging from Ray-vin.com (site is still up even tho Ray is retired) in the tech pages.
When you can buy a complete lower assembly for what the drills and cutters you would need to machine one (let alone the time) would cost, why bother? It's a neat project, just not something you are going to make money at unless you have the proper license.
As for the "criminals will do this" argument, no, probably not. It takes work, criminals by definition want something for nothing, or next to nothing, and are usually pretty lazy. Besides there are plenty of sources for better weapons that take less effort than building your own.