Because you've cut it, forming a thread conveniently leaves the root in compression which minimises the tendency to fracture. Plus if you look thread forming bolts tend to be 10.9 or 12.9, not "normal bolts". & as they've formed the thread it's undersize, so they self lock. You can just buy them from Ford, the ends are in free space so 2nd hand ones tend to be corroded past useful unless you find a fairly new van. Ideally you're never going to try loading them, but you'd really like them to hold if you do.