[Why is that? is it just because they are that common? or is it because they are that accurate?]
Accuracy has a lot to do with it, but IMHO it's mostly because they're easy.
Easy to bed, easy to set up, and a good trigger from the factory, all add up to the "smith" being able to spend less time/energy to turn out a "custom", ergo, earn a better "bottom line".
IMHO, its also why most M700 "customs" carry plastic stocks, ILO high grade, hand-carved, wood.
It's quite another thing, to get the square - bottomed Mauser 98/Winchester 70 bedding correct/just right - without resorting to a mud job(glass-bedding). Ditto, feeding and scope mounts.