That's because they are mounted poorly. Heat/cold cycles can loosen screws that don't have appropriate spring washers on them over time. Loctite can also prevent movement.
Spring washers are among the most useless things that can be used for prevent screw from loosening though. https://ntrs.nasa.gov/citations/19900009424
Nice reading. Well recommended.
Good doco. They mentioned nyloc nuts which have the greatest efficacy. It is a safety requirement in Australia that earthed chassis terminals have a nyloc nut securing the earth wire to the chassis and NO star washer between the lug and the chassis. I have also used nylocs on circuit boards where something must frame earthed.
Another point is that over time, metallic bonds are formed on threads and other mating metals over time, making them more diffucult to remove or shake loose.
A bloke came around here to buy my 30-year old 7' x 4' trailer earlier this year. The trailer had an oversized ball, which prevented theft. So he had to remove the 2 inch ball from his 4WD and put my 2.5 inch ball on. Trouble is the old ball had been left untouched in his 4WD for 25 years. To remove it, no amount of WD40 could shift it. I did not want to use a blow torch near a fuel tank or the greasy underside of his vehicle. A large pipe wrench with an extension handle could not budge it. What worked was putting the pipe wrench on the nut and backing the 4WD into a large power pole. The heavy duty pipe wrench handle was actually flexing. Eventually it released with an almighty BANG. Looking at the threads, I don't think rust played a big part. It was more likely the metallic bonds in the thread which had formed over 25 years. Anyone who has left metallic gauge blocks up against each other for a few weeks will know how strong these bonds can become.