I'll ask the diametrically opposed question.....
Why don't match grade bullets have a cannelure???..... and, why don't shooters in the pure accuracy games crimp their bullets???
Kinda' answers itself now.....
Actually, you should not rush to a conclusion on this.
Further, it depends on what you call a "crimp."
Heavily mashing the mouth of the case into the side of the bullet is one type of crimp.
Other times, a mild crimp is used on bullets that have no cannelure, and accuracy results are markedly improved. It depends on the load, and it depends on the rifle.
The purpose of the cannelure is generally to create a "soft spot" on the bullet for applying a heavy crimp, and in the case of military ammo, to provide a rough spot to hold case neck sealer.
Creating that soft-spot, or an indentation does not necessarily have any effect on the inherent accuracy of the projectile. This is especially true when the cannelure is produced by the original manufacturer.
Since match-rifle shooters may not be constrained by magazine-length for seating depth of the bullets, there is no point in the manufacturer putting a crimp on bullets to be sold to match-rifle shooters. How could the manufacturer possibly know where to place the cannelure, or whether a cannelure is even desired by the buyer?
This doesn't mean the absence of a cannelure makes the bullet any more accurate than a bullet, produced by the same manufacturer, that does have a cannelure.
In my experience, the presence of a cannelure on a first-quality bullet from a major manufacturer has a neutral effect on the accuracy of the bullet.
I shot a bunch of Remington 150-grain bullets with cannelure from my M1A in highpower rifle competition. I couldn't tell the difference at 100 yards between those bullets and the Sierra 168-grain MatchKing bullets. I did not crimp the Remington bullets. Hardly a "scientific" test... but still an anecdote worthy of note.