Suspension fasteners are not ordinary bolts. They are clamping a joint that moves under load, thousands of times, often with the whole weight of the truck arriving at once.
A bolt that has been torqued, loaded and backed off a few times has already given up some of its clamping ability, and a bolt hole that has been working slightly loose will not be obvious until something moves that should not. On a rear end you are taking apart anyway, fresh hardware is the cheapest insurance in the job.
It is also simply faster. Chasing individual fasteners across a parts counter costs more time than the kit costs money.
