Rough door openings, the frame into which the door jamb fits—the jamb being the part that the hinges screw into—are usually neither plumbed (i.e., made perfectly vertical)nor squared very precisely. It’s the nature of rough capentry, by which I mean all the wood that makes up the structure of a house and that lies behind the finished surfaces, to be, well, rough. Not terribly precise. It’s the job of finish carpenters to focus on details such as final plumbing and squaring, and they’re the ones who put in the door jambs to which the hinges get attached. And they should be trying to make the openings as plumb and square as possible so that things like doors operate with ease. So the finish carpenters fine-tune the door openings, thereby covering up the rough structure underneath and it is they who screw the hinges onto the wood that nicely finishes the opening.
This way of building has its roots at least partly in the assembly-line style of home construction that arose in this country, wherein a successive groups of workers come in to execute various phases of the building project. Finish carpenters are among the last of the groups on the job site. So that’s why, I think, those half inch screws get used.
Your door is probably a heavy, solid core door—as most exterior doors are. Also, likely rather than making your door “level,” you just made it fit the opening better by adding new screws that bite into the framing. Perhaps your door jamb was warped and/or poorly attached to the door frame underneath. Sounds like the crew that hung this door may have been cutting some corners