Maybe I've just never seen an upper far enough out of line in the few dozens I've seen.
How would you know? Unless you're analyzing bolt contact faces and/or broken bolts, you wouldn't know by just looking at them.
AR bolts are free to shift side to side and up/down a bit (concentricity), but can't do much to correct angular misalignment. A receiver face that's out of true changes the angle of the barrel and extension.
It's the same idea as truing the bolt lugs and receiver face in a bolt action; it doesn't guarantee better accuracy but is one step in making the system work right. Part of that is durability, not accuracy.
AR bolts are free to shift side to side and up/down a bit, but not so much to change angle. A receiver face that's out of true changes the angle of the barrel and extension. In most cases it's not enough to matter for most users, but if you're building a premium rifle and making sure everything's right, you'd true the receiver face in most cases.
