If this is a photo after 8 winters, then why do the untreated bolts not rust?
Because the material is different? Frame parts are probably painted and screws are electrolytically protected.
In addition, screw heads protrude and can dry more easily.
It often rusts where sand and mud collect. Because this keeps the sheet metal or steel moist.
You can easily test this by taking an untreated sheet of metal and wetting it with a water hose. If it hangs so that it can drip and dry, you can do this again and again for decades without the sheet metal rusting through. It will form surface rust but will not rust through.
On the other hand, take a painted metal sheet and put a pile of mud on it and leave the metal sheet outside. Despite the paint, the sheet metal will have rusted through in a few years, because the mud constantly retains moisture on the surface.
That's why it's important, among other things, to clean the cars underneath regularly with a high-pressure cleaner, especially the places where mud settles in the whell housings.
And it is very important to protect the cavities, because water draws in there simply through condensation, but it has difficulty drying out and as a result cavities often remain moist for a long time. Cars usually rust in places where mud gets in, or from the inside to the outside where there are cavities.