Here’s why: Your personal anecdote based on your experience of building and owning 3 AR’s and knowing at least one buddy with a AR is somehow supposed to be evidence that carrying a spare bolt is pertinent - akin to carrying a spare tire in your car. In today’s market, a quality spare bolt is ~$100, and there an estimated 25 million AR-15 owners in the US. So a spare bolt owned by all of us is a $2.5 BILLION - with a B - industry all of its own, just so we’d all have spare bolts, nothing else, just spare bolts.
As I mentioned above, assuming your buddy is like almost all AR-15 owners, the only consequence your buddy had when his bolt failed was that he found himself at a hobby range holding a “deadline” AR, and he had to go home and couldn’t play for 1-4 days while he sought a replacement (assuming there wasn’t a shop within reasonable driving distance of the range to buy a new one within minutes or hours). Pretty low consequence for that particular failure.
As a comparative anecdote of MY personal experience, I’ve owned something around 60-80 AR’s at best estimate of my own, and operated a business for several years based on building, rebuilding, and repairing AR’s. Not holding an FFL today, I still end up building around a one per month. I’ve built from ground up more than 200 AR’s, and have built and rebuilt more than 700 others in the last ~25yrs. I’ve used AR’s in high volume 3 gun competition (as well as Service Rifle competition, which was my introduction to the AR) and at least 10 “carbine courses” I can recall to count over the years - a couple of my current AR’s are PRS competition rifles now, so they see a pretty sizable volume of fire compared to the average AR owner. I have loaded at least 5,000 rounds per year for my AR’s since 2000, with some years topping 25,000. Several of my customers have been 3 gun and service rifle competitors, and dozens of them have taken my rifles through high demand training courses - when I held my FFL, I manufactured ammo to go with my rifles, and during my relatively short business in doing so, I sold over a million rounds to those customers. My “buddies” for most of my life have competed in shooting sports with AR’s, or make a living relying upon one. And through all of that - bolt breakage is NOT 1) a common failure mode, and 2) is NOT a high consequence failure for nearly all AR owners.
I can’t recall breaking more than 3 bolts in my personal use (none of which have been 6.5 Grendels, for reference), and I’m not certain I recall more than that many coming to my bench for repair/replacement. And for almost all of those breakages, the ultimate consequence is “I went home for the day and had to wait 1-4 days to get a new bolt.”
AR owners do LOSE a few specific things - extractor springs, ejector springs and ejectors, firing pin retainers, trigger/hammer pins, and there ARE parts which do break - but odds are, most AR owners will never experience a bolt breakage, never a firing pin breakage or a cam pin. I’m more prone to bet on folks damaging roll pins by using the wrong punches for removal and installation when they’re “servicing” their bolts, and WAY more prone to bet on folks bending charging handles than to bet on broken bolts… I’d bet I’ve had 20-30 bent charging handles come to my bench over the years.
I’ve had more people use the wrong buffer in their home builds and smash their carriers into their lowers than broken bolts. I’ve had more people fall with or otherwise drop their AR’s and break their lowers than I’ve seen in broken bolts (or buttsmashing a coyote). I’ve seen more broken bolt stops than broken bolts. I’ve seen broken hammers, and even broken hammer/trigger pins. Hell, I’d bet more people fire 300 blk ammo out of 5.56 barrels every year than break bolts. But as I stated in my first, unless your livelihood or competition performance depends upon an AR - in which case, you should have 2 or more AR’s ready for the task - then the consequence of breaking almost any AR part is simply ordering a new part on your phone while leaving the range for the day, and waiting 1-4 days to receive a replacement.
I know guys which have never built an AR which have spare pivot detent pins, because someone online said they were good to have around because they’re so easy to lose… how in Sam Hell is someone going to lose a detent pin from a pre-built rifle? They don’t even know how to take it out to be able to lose it, and wouldn’t ever need to do so… but they spent money to have the spare…
So no, I don’t tell my customers or my friends which only casually use their AR’s to waste money carrying inventory of spare parts. They have a spare tire under their truck beds because having a flat can have significant consequences - but they don’t have 4 spares on board. They might have spare oil filters on hand, maybe air filters even, as regular maintenance items, but they don’t have spare plug wires or spare fuel injectors, they don’t have spare shocks or struts, don’t have spare u-joint bearings, don’t have spare transmissions or engines sitting on the shelf…
It’s more sensible to balance your likelihood of risk with your consequence of failure, rather than wasting money on spare parts you’ll likely never use and would have no tangible difference in experience if you DID need to use them and simply ordered them later when you actually experienced a failure.