Loose draft caps

The doors are threaded as well. Some fabricators tacked the inner locking nut with weld. If you can loosen them, first remove inside nut, then remove bolt with draft cap from door. See if the draft cap has a steel insert cast in it. Some are threaded aluminum which wears if not greased. If a magnet sticks to the center showing it has a steel insert, the threads wear about 50 /50 male and female, so replacing the bolt or plating it takes up half the thread clearance. When installing, adjust bolt length so you can get on the head of bolt with wrench or socket with draft cap closed. When adjusted too far in, it is difficult to get on the bolt head with anything but a very thin walled socket. Keeping silver anti-seize on the bolt threads prevents wear. Opening draft cap fully allows a brush to reach through the back of door opening to grease threads. Keep it on the hinge pins too. They are the only moving parts to wear out.

The caps normally seal well enough to kill the fire.
The bolts can get bent slightly or have a door drilled and tapped slightly crooked to require tweaking the bold sideways if the cap edge doesn't make contact all the way around.
Can't answer why your door is not threaded. They normally are. Saving time threading and tap cost?? Once the bolt is mounted to the door it doesn't matter if the door is threaded when jam nuts are tight on each side of the door.

Probably for strength of the wall of the intake support. I can imagine fabricators hiring help that doesn't know how to sharpen a half inch drill bit to prevent wobble of an off center bit, increasing the hole size until the support walls are too thin. I'm amazed there is not a demand for repairing or replacement of doors due to broken intake bolt supports.
We're used to seeing things built to minimum standards today. Back then things were built for longevity, not for speed of assembly.