Because they wouldn't need to shut down factories if they had bigger stock of parts on site.
The order for a seat or mirror or door handle doesn't exist at the tier 1 suppliers where my equipment gets installed until 10 hours before it is needed. At that point, my PLC gets a request to induct a particular part number and paint color into the manifest, inducts it and acknowledges that request, the machine builds it, laser-marks a serial number onto it, tests it, uploads the test results with serial number to the database, packs it out into a foam-lined box or a dedicated nest and uploads the box barcode to the database, that box gets loaded onto a pallet, the pallet into a trailer, the trailer drives 20 minutes away to the OEM, and the trailer gets unloaded (in reverse order; truck size and the resulting last-in-first-out queue is one of the main drivers for JIT lifetimes) so the parts land at the assembly line just a couple hours before the vehicle they belong to is ready to receive them.
You'll notice that this does not leave a lot of wiggle room for anything to go wrong. When I'm physically a 5 hour drive away from the plant, it's not like I can drive to the site, debug a wiring issue, order a replacement widget online - even by UPS Next Day Air Early A.M. - and then install it on-site before holding up literally millions of dollars worth of production. And my little mirror line is just one of hundreds of parts that need to arrive on time.
Hurricanes, blizzards, train/truck worker strikes, cargo ship stuck in a canal, etc. For a multinational, those are not black swan events, something of that nature is going to break the pipeline on a regular basis. Potentially for days of backlog.
Clearly it must mostly work, but that is some crazy thin margin. I assume whoever works logistics is just perpetually on the edge of a heart attack.