Interesting how a simple question can get hijacked.
Anyway, they do make all-steel doors, you just have to go to a place that sells them, probably a place that specializes in doors. They're commonly used in steel and in concrete buildings.
Even with a steel-framed door, if it's not installed properly it can be easily kicked in. The weak point, even with a wood door, is the attachment of the door hinges and deadbolt strikers to the studs. It's important to use long, heavy screws for the hinges and deadbolt; there are usually a couple supplied with the door and couple with the lockset. These screws need to go all the way through the door frame and well into the stud. If the rough framing is oversize, shim it with a board so the screws have more material to grab. You shouldn't rely on the door frame itself, because it's only as strong as its attachment to the house.