[john walker's workshop]

it's probably a matter of them freezing in the head, like studs do. at least with studs, you can heat the nut and remove it, and it's fine if the stud just stays in the head like it's supposed to do. with a frozen bolt, there's no way to heat it, and it would twist off, requiring a lot of drilling, tapping, inserting, and often ruining the head in the process. not that studs don't break, but it's not that often.