Simply that calipers as far as precision tools go are the least precise but cheap and handy to use.
$120 for a set might seem really expensive to folks that only own tape measures but when compared to a $500 bore micrometer or a $12,000 set of micrometers, they are cheap and not as accurate.
If you want precision measurements you use precision tools.
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Calipers are “accurate enough” for most reloading chores, meaning more precision isn’t necessary for them.
You could measure your, say seating depth to .000002 but it wouldn’t matter because you can already tell they are not that precise just using your calipers...